GOP Rep. Matt Rosendale announced Friday that he had filed for Senate in Montana, setting up a contentious primary against national Republicans’ preferred pick to take on vulnerable Democratic Sen. Jon Tester.

Rosendale, a member of the hard-line House Freedom Caucus who lost to Tester in 2018, will be up against former Navy SEAL Tim Sheehy, who announced his campaign in June and already has the backing of the National Republican Senatorial Committee, led by Montana’s senior senator, Steve Daines.

Rosendale’s candidacy also goes against the wishes of Senate Minority Leader Mitch McConnell, who is supporting Sheehy in a race seen as critical to Republicans’ efforts to flip the chamber.

“We’ve made great accomplishments in the House, only to see them die at the hands of Mitch McConnell and Chuck Schumer in the Senate,” Rosendale said on social media Friday as he officially announced his run.

Hours later, former President Donald Trump dealt a blow to Rosendale’s nascent campaign by endorsing Sheehy, whom he praised as “an American Hero and highly successful Businessman.”

“I also respect Matt Rosendale, and was very happy to Endorse him in the past - and will Endorse him again in the future should he decide to change course and run for his Congressional Seat,” Trump wrote on social media. “But in this instance, Tim is the candidate who is currently best-positioned to DEFEAT Lazy Jon Tester, and Regain the Republican Majority in the United States Senate.”

Both Sheehy and Rosendale have endorsed Trump’s presidential run.

The Montana contest will be a test for national GOP leaders, who are taking a more active role in primaries this cycle than in the past, when flawed nominees have sometimes cost them seats.

Daines on Friday hit Rosendale for jumping into the primary, fueling concerns about the party’s ability to flip the seat in November.

“It’s unfortunate that rather than building seniority for our great state in the House, Matt is choosing to abandon his seat and create a divisive primary,” Daines said in a statement, while reiterating his support for Sheehy.

“Whichever party wins the Montana Senate seat will control the United States Senate in 2024, and Republicans cannot risk nominating a candidate who gave Jon Tester the biggest victory of his career,” he continued.

First elected to the US House in 2020, Rosendale has been a thorn in the side of GOP leadership. He was one of the eight House Republicans in October who voted to oust Kevin McCarthy as speaker. He also initially opposed McCarthy’s speakership bid in January 2023, before ultimately voting “present” to allow McCarthy to clinch the gavel on the 15th ballot.

The current House speaker, Mike Johnson, informed several Republicans on Thursday that he would not be endorsing Rosendale for Senate. The move came after reports that he was preparing to back his House colleague prompted swift blowback for Johnson. Rosendale backed Johnson for speaker last fall.

Tester is one of the most vulnerable Democrats up for reelection in 2024 as he runs for a fourth term in the red state.

His campaign has been on the Montana airwaves with positive bio ads, reminding voters of his farming background and his commitment to public lands, Second Amendment rights and defending Montanans’ way of life.

According to his most recent filings, Tester ended December with more than $11 million in cash on hand. By contrast, Sheehy ended the year with more than $1.2 million in the bank. Rosendale had nearly $1.7 million banked in his House campaign account as of December 31. He would be able to transfer those funds for use in his Senate bid.

Rosendale previously served in the Montana Legislature and as state auditor. He moved to Montana in 2002, and the former land developer’s Maryland roots were a frequent target of Democratic attacks in his 2018 Senate race against Tester, which the incumbent won by less than 4 points.