With a little more than a year to go until Election Day 2024, the Senate battlefield is evolving, but slowly.

Democrats continue to cling to a 51-49 majority in the chamber, with three of their incumbents from red states – Sherrod Brown in Ohio, Joe Manchin in West Virginia, and Jon Tester in Montana – preparing to face the voters with their party’s majority in the balance.

In our second handicapping of next year’s key Senate races – the first was published in early June – the contours of the battle for the majority remain in place, although in a few races new developments have produced some new wrinkles.

In Pennsylvania, the entry of Republican Dave McCormick – seemingly without significant primary competition – has upped the competitiveness of the reelection race for Democratic Sen. Bob Casey. In Montana, Republican leaders like the look of political newcomer Tim Sheehy, though he may yet face primary competition before securing the right to challenge Tester. In Michigan, the entry of former Rep. Mike Rogers into the race cheered the GOP establishment, but – like Sheehy – he may not get a free ride in the primary.

Meanwhile, in Arizona, MAGA star Kari Lake’s Senate candidacy moves the race closer to a volatile, three-way slugfest, assuming independent Sen. Kyrsten Sinema chooses to run again.

Then there’s New Jersey, where Democratic Sen. Bob Menendez has been indicted. Menendez has refused to step down and will now face at least one serious primary challenger. While the seat should ultimately remain in Democratic hands, that’s not a guarantee if Menendez manages to survive the primary.

As has been the case for the entire election cycle, Democrats are defending the chamber’s eight most vulnerable seats (assuming you count Arizona, where Sinema, a former Democrat, has continued to caucus with her old party even though she’s formally switched her party affiliation).

Republicans, for their part, are mostly playing offense. They are defending only two seats that look even remotely competitive: Texas and Florida.

Here’s a state-by-state rundown of the Senate battleground for 2024. The seats are listed in descending order from those most likely to go Republican to those most likely to go Democratic. Aside from adding New Jersey to the Likely Democratic category, the only shifts we’ve made since June are within categories. We’ve reordered the Toss-Up category so that Ohio is now the closest to Lean Republican and Arizona is the closest to Lean Democratic, with Montana right in the middle. And within the Lean Democratic category, Nevada remains closest to a Toss-Up, but now Pennsylvania and Michigan are next, while Wisconsin shifts closest to Likely Democratic.

Indiana (open seat), Missouri (Josh Hawley), Mississippi (Roger Wicker), North Dakota (Kevin Cramer), Nebraska (Deb Fischer), Nebraska special election (Pete Ricketts), Tennessee (Marsha Blackburn), Utah (open seat) and Wyoming (John Barrasso).

A word on Utah: Republican Mitt Romney has decided to retire from the Senate amid his high-profile disagreements with the direction of his party.

Before he announced his retirement, Romney was already facing the prospect of a challenge from his right by Riverton Mayor Trent Staggs. Utah House Speaker Brad Wilson, a more establishment choice close to Romney, is now expected to run as well. But other Republicans could enter the race, too. Democrats, meanwhile, have a threadbare bench in Utah and are not expected to be a factor.

In their bid to oust Republican Sen. Rick Scott, Democrats have found their preferred candidate: former Rep. Debbie Mucarsel-Powell, who served one term representing a Miami-area district but who lost in 2020 amid a Republican wave in the region and the state. Scott has some vulnerabilities as a candidate – he’s feuded with Senate Minority Leader Mitch McConnell over campaign tactics and released a policy manifesto that initially called for cuts to Social Security and Medicare – but he’s an incumbent, wealthy and well-known from his two terms as governor.

And while Democratic leaders are glad to see Mucarsel-Powell in the race, they haven’t succeeded in clearing the field for her. Other candidates include retired Navy Cmdr. Phil Ehr, former Rep. Alan Grayson and former state Rep. Brian Rush. Also reportedly considering a run as a Democrat is Stanley Campbell, a rocket scientist, Navy pilot, golf course owner and brother of Luther Campbell, the former 2 Live Crew rapper.

“If the Democratic primary gets heated and expensive, that could weaken Mucarsel-Powell’s general election standing,” even if she wins the primary, says Sean D. Foreman, a Barry University political scientist. Further complicating the challenge, Foreman said, is that Florida has lost its status as a top-tier presidential swing state, likely robbing down-ballot Democrats of the benefits of presidential campaign resources in the state.

Even if Florida Democrats rebound in the 2024 presidential year after a broad collapse in 2022, when polarizing Republican Gov. Ron DeSantis won reelection by nearly 20 points, Scott remains a strong favorite to win another term. This race remains Likely Republican.

The Senate race in Texas is looking a little bit like the one in Florida: a Republican incumbent who might be vulnerable, a credible Democratic front-runner for the nomination, but a potentially messy primary looming for the challenging party.

Republican Sen. Ted Cruz’s prickly persona, his hard-line policy views, and Texas’ modest tracking to the ideological center in recent election cycles puts him in some jeopardy. In 2018, Cruz narrowly avoided being ousted by the Democratic nominee, then-Rep. Beto O’Rourke. But despite incremental Democratic gains in recent elections, Texas has not elected a Democratic senator since 1988.

The front-runner for the Democratic nomination in 2024 could be even stronger than O’Rourke: Rep. Colin Allred, a charismatic former NFL player who won a tough House race in a competitive district in 2018.

The challenge for Democrats is a competitive primary. The Democratic Latino political elite is rallying behind state Sen. Roland Gutierrez, who is running to Allred’s left and has been the most visible Democrat advocating for gun control. (He represents Uvalde, the site of a major mass shooting in 2022.) Also running on the Democratic side are state Rep. Carl Sherman and Mark Gonzalez, the former district attorney of Nueces County, which includes Corpus Christi.

The primary could become divisive, and the large field makes it “very likely that the contest will go to a May runoff, with neither Allred nor Gutierrez able to win more than 50% in the March primary,” says Rice University political scientist Mark P. Jones. Whoever emerges as the nominee will be “at least somewhat damaged,” Jones said. They will also have an empty campaign warchest, although rank-and-file Democratic donors should be able to refill it quickly if it looks like the nominee has a real shot at ousting Cruz, a bete noire for Democrats.

For now, the contest remains Likely Republican.

Manchin – a Democrat representing deep-red West Virginia, where Trump won by 39 points in 2020 – hasn’t officially declared that he’s running for a new term. But it’s widely assumed that if he does, he’ll face a stiff challenge – and if he doesn’t, the Republicans are certain to flip the seat.

Polling suggests that Manchin’s reelection hopes rest on the outcome of the GOP primary, which pits Gov. Jim Justice against Rep. Alex Mooney. Justice, until recently a billionaire, has deeper pockets and a down-home populist persona. He’s the favorite of national Republicans. Mooney, for his part, can rely on financial backing from the conservative Club for Growth. Justice, who was initially elected governor as a Democrat, has occasionally broken with GOP orthodoxy, while Mooney previously lived in Maryland, unlike Justice, who is West Virginia born and bred.

A MetroNews West Virginia Poll in August found Justice leading Manchin in a general-election face-off, 51%-38%. But the poll found Manchin leading Mooney, 45%-41%. The poll found Justice a heavy favorite in the primary, leading 58%-26%.

If Justice notches a primary victory and faces Manchin, this race could shift to Likely Republican. If Manchin decides not to run, it would become Safe Republican. For now, though, we’re keeping it at Lean Republican.

Brown is a skilled politician who has won three Senate terms in a state where his blue-collar oriented, pro-worker pitch has historically found a receptive audience. But Ohio has turned increasingly red.

“Brown is about to face something he’s never faced before: a credible opponent combined with a strong GOP tide in the state,” says Mark Weaver, a longtime Republican observer in the state. Biden’s expected loss in Ohio will make it harder for Brown to separate himself from the Democratic label, Weaver adds.

With a wide-open GOP primary, the identity of Brown’s opponent is unsettled. State Sen. Matt Dolan, a relative moderate who finished a distant third in the 2022 Senate primary, and Bernie Moreno, a businessman from the Trump wing of the party, are both able to self-fund. Ohio’s elected secretary of state, Frank LaRose, is also running, though he took a hit when a ballot measure he backed earlier this year that could have made it easier to ban abortion in the state lost decisively, despite the state’s conservative leanings.

Republicans say they’re confident that any of the three can defeat Brown. Recent independent polls have shown Brown with a modest edge, but in this political environment, he can’t take anything for granted. Pending further developments, we’re keeping this race in the Toss-Up category.

Tester is the lone statewide Democratic officeholder in Montana. As with West Virginia and Ohio, Montana backed Trump for president twice. He’s succeeded in the past thanks to his down-home style and political acumen, but the state is now arguably redder than ever.

National Republicans like the political newcomer who recently joined the race: Tim Sheehy, the founder and CEO of Bridger Aerospace and a retired Navy SEAL. He has the blessing of the state’s other senator, Republican Steve Daines, as well as Montana Gov. Greg Gianforte, Montana Rep. Ryan Zinke and Senate Minority Leader Mitch McConnell. Observers say Sheehy has a strong profile but will need on-the-job training if he is to oust Tester, who’s survived in difficult political territory before.

But to the chagrin of some Republicans, Sheehy may not have a clear path to the nomination, if Rep. Matt Rosendale gets into the race. Rosendale irritated some Republicans in 2018 when he lost what they considered a winnable race against Tester by more than 3 points. In addition, the deep-pocketed Club for Growth has indicated that they won’t be supporting Rosendale again as they did in 2018. Rosendale has until the filing deadline in March to decide whether to run.

Until the GOP nomination is decided, this race is a pure Toss-Up.

Sinema was elected as a Democrat in 2018, but after a stormy period as a key but unreliable vote for Biden’s agenda, she became an independent following the 2022 elections, although she continues to caucus with Democrats, enabling the party to keep the Senate majority.

Sinema’s frosty relations with the Biden administration and her often eccentric political style have soured Democrats on her, and if she runs again – something she hasn’t yet confirmed – she would face serious competition from both parties.

The only major Democrat in the race, Rep. Ruben Gallego, has a historically liberal voting record, but he’s also a Marine veteran who saw combat in Iraq, and he’s expected to run toward the center to court Republicans and independents disaffected with the state GOP’s shift to the right.

On the Republican side, Kari Lake has officially entered the race. Lake is a former television newscaster who lost a bid for governor in 2022 after becoming a darling of the Trump wing of the party by focusing on election fraud. While Lake is considered a talented politician, her positions are viewed as toxic beyond the hard-core GOP base. In the primary, she’ll face Pinal County Sheriff Mark Lamb. Sheriffs tend to run well in Arizona, but he is little known statewide.

A Noble Predictive Insights poll from July found Gallego leading a three-way race with 34%, against 26% for Sinema and 25% for Lake. The margin was similar if Lamb won the nomination instead.

”Sinema stole voters from Lake and some from Gallego,” says the firm’s founder and managing partner, Mike Noble.

“Arizona Republicans are stuck in a tough bind,” Noble adds. “MAGA Republicans like Kari Lake can win primary elections, but in recent years they’ve struggled to seal the deal with the broader electorate. It’s still early and anything can happen. But Democrats must be happy that Lake has entered the race.”

With many permutations in this race still to be worked out, we’re keeping it in the Toss-up category.

In closely divided Nevada, first-term Democratic Sen. Jacky Rosen is looking ahead to a competitive reelection contest.

“Nevada, a state flush with white and Latino working-class voters, seems like a natural target for the new, populist GOP,” says Noble, who also polls in Nevada.

Just how competitive the race becomes will depend on the GOP nominee.

Sam Brown, a wounded Army veteran who lost the 2022 primary for Nevada’s other Senate seat, is running again. Even though Brown lost the primary to Adam Laxalt (who in turn lost to Democratic incumbent Catherine Cortez Masto) Brown made a strong enough impression that he’s the preferred candidate for Republican leaders.

At least two of the other GOP candidates in the primary are from the wing of the party firmly aligned with Trump. One, Jim Marchant, won the Republican nomination for secretary of state in 2022. However, his support for election conspiracies helped the Democratic nominee win by about 2 points. The other is Jeff Gunter, who Trump named as U.S. ambassador to Iceland.

Observers see Rosen as the favorite against either Marchant or Gunter, while a Rosen-Brown matchup would be close, hard-fought and expensive.

Until the primary plays out, this contest remains in the Lean Democratic column.

One of the contests that appears tighter than it did a few months ago is Pennsylvania, where Casey is up for reelection. That’s for one reason: the emergence of a credible and deep-pocketed Republican challenger, McCormick.

Casey is iconic in Pennsylvania, having served three terms in the Senate and inheriting goodwill for his late namesake father, who served as governor. In the Senate, Casey hasn’t made any obvious mistakes that would put him at risk of winning a fourth term in 2024, but because Pennsylvania is one of three states – along with Wisconsin and Michigan – that backed Trump in 2016 before flipping back to Biden in 2020, this race has to be considered competitive.

Doug Mastriano, the far-right candidate who lost Pennsylvania’s 2022 gubernatorial election by nearly 15 points, has opted not to run, leaving the field to McCormick, a former hedge-fund executive who lost the 2022 Senate primary to television doctor Mehmet Oz, as the expected GOP nominee.

McCormick’s establishment credentials could enable him to make inroads with moderate GOP voters who disliked Mastriano hard-right positioning. But Democrats are looking to attack McCormick on abortion, where he ran to the right in his primary against Oz, and the arguably tenuous strength of his ties to the state, an issue that hobbled Oz in the 2022 Senate contest.

Polls taken before McCormick’s official entry showed Casey with a mid-single-digit edge. But that should close as the race develops, especially if the presidential contest in the state remains tight. For now, we’re keeping this race at Lean Democratic.

This is an open seat, due to the retirement of Democratic incumbent Debbie Stabenow.

Aided by abortion politics and the state GOP’s nomination of extreme candidates, Michigan Democrats had a successful election season in 2022, easily winning the governorship and top statewide executive offices and flipping control of both chambers of the legislature. Democrats are hoping to ride this momentum again in 2024.

The Democratic frontrunner is Rep. Elissa Slotkin, who has won three straight elections in competitive districts, running as a moderate with foreign policy experience. Slotkin is considered a strong candidate for the general election, but first she will have to survive a contested primary without sustaining too many blows. In the primary, Slotkin is poised to face State Board of Education President Pamela Pugh, former state Rep. Leslie Love, actor Hill Harper, Arab American civil rights activist Nasser Beydoun and environmental lawyer Zack Burns.

The GOP field is less settled. The biggest development is the entry of former seven-term Rep. Rogers.

“If he can get through a primary, Rogers would be the best nominee the GOP could hope for,” says Bill Ballenger, publisher of the Ballenger Report, a newsletter on Michigan politics.

Former Detroit Police Chief James Craig, who tried but failed to make the gubernatorial primary ballot in 2022, is also running. Also considering a Republican bid is Peter Meijer, a retail scion who served in the U.S. House before losing a primary to a Trump ally in 2022 (a history that could hobble him in the GOP primary), and businessman Perry Johnson, who has been spending the past year running for president without qualifying for the debate stage. Several other less prominent Republicans are already in the race, including Nikki Snyder, an elected member of the State Board of Education.

Slotkin would have the edge in the general election, although a face-off with Rogers, if it were to happen, could be competitive, especially as the two spar over their long records in foreign affairs. The Democrats start this race with an edge.

While Wisconsin is arguably a tougher state for Democrats to win than Nevada, Pennsylvania or Michigan, it seems to be the one state of the four where the party is best positioned in this year’s Senate races.

That’s partly due to the battle-tested record of incumbent Tammy Baldwin, who won by a surprisingly wide 11-point margin in 2018. And it’s partly due to the momentum Wisconsin Democrats showed in a 2023 Supreme Court race in which the more liberal candidate won by about 11 points.

But perhaps most important is the unsettled Republican primary field. GOP Rep. Mike Gallagher and Rep. Tom Tiffany have decided to pass on the race, so Republicans are scoping out banker Eric Hovde, or perhaps businessman Scott Mayer. Neither Republican has attracted much enthusiasm on the GOP side, however. And many Republicans would be concerned if former Milwaukee County Sheriff David Clarke, an outspoken Trump ally, were to get in the race and win the nomination.

Due to Wisconsin’s closely divided politics, this contest hasn’t progressed beyond Lean Democratic. But if Republicans can’t lure a credible candidate into the race, it could eventually shift to Likely Democratic.

The federal indictment of Democratic Sen. Bob Menendez – with all its details of gold bars and stacks of cash – has upended what should have been a straightforward reelection.

Menendez has refused to resign or announce his retirement, but after losing the backing of key home-state Democrats – including Gov. Phil Murphy, fellow Sen. Cory Booker and most of the Democratic House delegation – it’s become clear that his party won’t be supporting him electorally this year as they did after Menendez’s 2015 indictment, which eventually led to a hung jury.

One strong challenger has already emerged: Rep. Andy Kim, who is someone a broad swath of the party could support in a primary against Menendez. The risk for the party is that additional Democrats could enter the race, effectively lowering the threshold to win and raising the likelihood of Menendez squeaking by in the primary. One possibility of another primary candidate is New Jersey first lady Tammy Murphy. Rep. Mikie Sherrill, another potentially strong candidate, has said she would not run for Senate.

On the GOP side, Rep. Jeff Van Drew, a Republican originally elected as a Democrat, is reportedly considering a bid. But Republicans have lost every Senate election in New Jersey since 1972, so their main hope of winning could come from Menendez securing renomination. That’s enough of a risk for Democrats for us to move this seat from Safe Democratic to Likely Democratic.

California (open seat), Connecticut (Chris Murphy), Delaware (open seat), Hawaii (Mazie Hirono), Massachusetts (Elizabeth Warren), Maryland (open seat), Maine (Angus King, independent who caucuses with Democrats), Minnesota (Amy Klobuchar), New Mexico (Martin Heinrich), New York (Kirsten Gillibrand), Rhode Island (Sheldon Whitehouse), Virginia (Tim Kaine), Vermont (Bernie Sanders, independent who caucuses with Democrats) and Washington state (Maria Cantwell).

Two of the three Democratic open seats are expected to be solidly Democratic in the general election but will involve spirited Democratic primaries. (Delaware does not look like it will have a competitive Democratic primary.)

Here’s how the primaries in California and Maryland are shaping up.

California Gov. Gavin Newsom has tapped Laphonza Butler, president of EMILY’s List, a Democratic political fundraising group, to succeed the late Democratic Sen. Dianne Feinstein.

Butler won’t run for a full term in 2024, but there were already three major Democrats running to succeed Feinstein, who had announced her retirement before her death. All three are Democratic House members: Barbara Lee, Katie Porter and Adam Schiff. Schiff and Porter hail from the Los Angeles area, while Lee, like Feinstein, is from the San Francisco Bay area. Schiff, who gained a national following from his role in Trump’s impeachments, is considered a modest front-runner.

One factor that could shape the primary is California’s unusual electoral system, in which the top two finishers from any party face off in the general election.

Former Los Angeles Dodger Steve Garvey is running as a Republican. If he manages to make the runoff, whoever is the highest finishing Democrat in the primary would be heavily favored in the runoff.

But there’s a decent possibility that two Democrats could win both top slots.

“The speculation now is on whether we will get an all-Democratic general election, most likely Schiff against Porter, or if Republicans are able to coalesce behind a single candidate to break that up,” says Marcia L. Godwin, a professor of public administration at the University of La Verne.

Either way, another Democratic win in November seems assured.

The Democratic primary to succeed retiring Sen. Ben Cardin has thinned out over the last few months as Rep. Jamie Raskin, Baltimore County Executive John Olszewski and Montgomery County Council Member Will Jawando have stepped back from running.

That has left two heavyweights: Angela Alsobrooks, the county executive of Prince George’s County, a populous, D.C.-adjacent suburb that is majority Black, and Rep. David Trone, a wealthy businessman. Former telecom executive Juan Dominguez is also running.

Alsobrooks has received support from key party figures in the state, including Gov. Wes Moore, and about 40 percent of the Democratic primary electorate is Black, like Alsobrooks. But Trone’s deep pockets all but assure that he will be a tough opponent. One of the two is almost certain to take their place in the Senate in January 2025.