Pennsylvania Sen. John Fetterman on Thursday endorsed Rep. Andy Kim in his Senate campaign against New Jersey first lady Tammy Murphy, becoming the first one in the upper chamber to weigh in on one of the country’s closely watched Democratic primaries.

For Kim, the backing by Fetterman adds a well-known name to a growing roster of elected officials supporting what is by some key measures an uphill challenge to reach the Senate. Though the third-term House member is a prolific fundraiser who is well-respected by his colleagues, Murphy has secured critical support from leaders of New Jersey’s Democratic-rich counties that is likely to translate to preferential ballot placement in the primary.

In an interview with POLITICO, Fetterman called New Jersey’s unique ballot-design system “the same kind of regime that had protected that kind of sleazeball Menendez,” but added that “I think Democratic voters are also savvy enough to understand what’s at stake here, and that they are going to want their voice fully reflected in D.C. And that voice is going to be Representative Kim’s.”

Fetterman, who was also the first sitting senator to call for Menendez’s resignation following his indictment last fall, also wrote in his endorsement announcement, “I feel very secure knowing that he has no gold bars underneath his mattress and won’t ever be accused of being a foreign agent for Egypt or Qatar.”

Kim and Murphy are both seeking to replace Menendez and have called for him to resign following the allegations he accepted cash, gold bars and a luxury vehicle in exchange for favors to benefit businesspeople and the government of Egypt. He and those businesspeople, as well as his wife, have pleaded not guilty. But Menendez was subsequently accused by federal prosecutors of using his influence as chair of the Senate Foreign Relations Committee to help Egypt and Qatar.

Fetterman also cited Murphy’s switch from the Republican Party to the Democratic Party in 2014 as a consideration, saying that he believes Kim would be a sure Democratic vote if elected to the Senate.

“You have the opportunity to have a choice between two people, as a Democratic voter,” Fetterman said. “You don’t have to wonder, well, does this person maybe or maybe not deliver those critical votes?”

Fetterman has already sent out an email to fundraise for Kim, and said that he intends to continue to use his platform to raise money and promote events. Kim also received the backing Thursday of six House Democrats from Pennsylvania: Dwight Evans, Madeleine Dean, Chrissy Houlahan, Mary Gay Scanlon, Susan Wild and Brendan Boyle. And in New Jersey, he received endorsements from the Democratic Coalition of Ocean County as well as seven Democratic municipal committees and eight local Democratic clubs.

Fetterman said he has nothing personal against Murphy, but he compared her past party affiliation to his 2022 GOP opponent in the Pennsylvania Senate race, Dr. Mehmet Oz, who has a home in New Jersey.

“I got tired of dealing with the Republican from New Jersey in my race, and now we have the opportunity to bring a reliable Democratic vote here to the Senate,” he said.

Alex Altman, communications director for the Murphy campaign, wrote to POLITICO, “Tammy is laser focused on listening to New Jersey voters and traveling the state to earn their support. Long before running for office, Tammy has worked to strengthen the Democratic Party in New Jersey to advance issues critical to working families.”

Murphy has racked up endorsements from seven Democratic county chairs, mostly in the northern part of the state that is more densely populated with Democratic voters. County endorsements play a major role in seeking the so-called county line, which brackets their preferred candidates together and often provides an advantage to candidates in primary races. And since New Jersey has not sent a Republican to the Senate in about half a century, whoever wins the primary is almost certain to win the general election in November.

Menendez has not announced whether or not he will run for reelection in 2024, but aide Jason Tuber told POLITICO on Thursday that Menendez will not participate in the Monmouth County Democrats’ convention seeking its endorsement for the primary. Murphy lives in Monmouth but that does not mean she is a lock for that county’s endorsement. Still, Tuber said “the Senator will not be filing for [Monmouth County Democrats] endorsement given the process is likely to be predetermined” citing statements from local Democratic leaders who have called for his resignation.

Daniel Han contributed to this report.