GOP Rep. Tony Gonzales of Texas is fighting to avoid defeat in a hotly contested Republican primary runoff Tuesday, with a challenger attacking him for taking bipartisan votes, including helping pass a new gun law in the wake of the school shooting in Uvalde in 2022.

Voters are heading to the polls in Texas’ 23rd Congressional District, a massive U.S.-Mexico border district that stretches from the outskirts of San Antonio to El Paso, to choose between Gonzales and conservative internet personality Brandon Herrera.

Gonzales took over 45% of the vote in the March primary and Herrera took only 25%, but the two advanced to a one-on-one runoff under Texas law because no one got majority support. And observers say the race has tightened considerably since then, with Gonzales in real danger of losing. He would be the first House member to lose a primary in 2024, outside one race that pitted two incumbents against each other after court-ordered redistricting in their state.

The primary runoff has drawn over $5 million in ad spending, according to AdImpact, with groups like the Republican Jewish Coalition Victory Fund, the American Action Network (a nonprofit aligned with House GOP leadership) and the Hispanic Leadership Alliance collectively spending millions to defend Gonzales.

Sam Markstein, the communications director for the RJC, told NBC News that the group views Gonzales as a “workhorse” in Congress and called Herrera “crazy.”

Herrera’s campaign has spent just over $700,000 on ads in the district since the primary, touting himself as a “gun rights advocate” and an ally of former President Donald Trump. He’s also attacked Gonzales for “[putting] American last.”

Herrera’s staunch support for gun rights raises an issue Gonzales has been fighting politically since 2022, when he joined 13 other Republican House members in voting with Democrats to pass the Bipartisan Safer Communities Act, a law that expanded background checks for gun purchasers under 21 and made it a federal crime to traffic illegal firearms in the U.S.

The bill passed the House just weeks after a deadly mass shooting killed 21 students and teachers at Robb Elementary School in Uvalde, which is located in Gonzales’ district.

The Texas Republican Party cited Gonzales‘ vote in favor of that bill as one reason that it chose to censure the incumbent last year.

Despite the political blowback, Gonzales told CBS’ “Face the Nation” on Sunday that “you cannot be afraid to do what you think is right. You’re never gonna get it perfect, but you have to fight for the things that are important to us. Keeping our kids safe from being murdered from crazy people, that should be important to all of us.”

“I’m not afraid of that vote. I’m not afraid of this runoff,” Gonzales added, a few minutes after he told moderator Margaret Brennan that, “I’m in the middle of a runoff fighting for my life, politically.”

Gonzales’ allies have hit Herrera on the airwaves with clips from his online videos, including one in which he mocked veteran suicides by saying, “I often think about putting a gun in my mouth. So I’m basically an honorary veteran.”

It’s an argument the pro-Gonzales groups hope will be particularly salient in this district, which is home to Joint Base San Antonio and Fort Bliss.

Gonzales’ allies have also tagged Herrera as a carpetbagger, saying he only recently moved to Texas from North Carolina.

Gonzales’ campaign did not respond to multiple requests for comment, while Herrera’s campaign declined to comment for this article.

Gonzales isn’t alone in getting outside support in the race. Prominent right-wing members of Congress have jumped in to endorse against their colleague, with Herrera touting support from GOP Reps. Matt Gaetz of Florida and Bob Good of Virginia. Good is also the chair of the House Freedom Caucus. Good is facing his own primary challenge in June, with the RJC spending against him as well, following a recent “no” vote on aid to Israel.

Officials at the RJC tied Herrera to the current “chaos” in Congress. “A very slim [GOP] majority, as you know, would be even more compromised and even more difficult by adding someone who would just go there to cause chaos,” Markstein said.

“We don’t need chaos in the House,” Markstein added, “We need strong leadership and effective leadership.”

Gonzales, meanwhile, touts backing from Texas Gov. Greg Abbott, House Speaker Mike Johnson, R-La., and even actor Matthew McConaughey, who recorded robocalls for Gonzales.

Gonzales’ fight for another term is the highest profile in a handful of primary runoff elections for Texas congressional seats. Abbott has also backed state Rep. Craig Goldman in his GOP runoff contest against business owner John O’Shea in Texas’ 12th District, an open seat currently represented by retiring Rep. Kay Granger.

And in Texas’ 28th District, retired Navy officer Jay Furman and rancher Lazaro Garza are headed to a runoff for the chance to face Democratic Rep. Henry Cuellar in November.

In May, Cuellar and his wife were indicted on federal charges of bribery and money laundering. The congressman has denied any wrongdoing.

This article was originally published on NBCNews.com