In a surprising move, GOP Representative Matt Rosendale has filed for Senate in Montana, setting the stage for a contentious primary against the National Republicans' preferred pick to take on vulnerable Democratic Senator Jon Tester. Rosendale, a member of the hardline House Freedom Caucus, previously lost to Tester in 2018.

Rosendale's entry into the race pits him against former Navy SEAL Tim Sheehy, who announced his campaign in June and already enjoys the backing of the National Republican Senatorial Committee, led by Montana's senior senator, Steve Daines. However, Rosendale's candidacy goes against the wishes of Senate Minority Leader Mitch McConnell, who is supporting Sheehy.

Announcing his run on social media, Rosendale expressed his frustration with the lack of progress in the Senate, saying, "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."

Hours later, former President Donald Trump dealt a blow to Rosendale's campaign by endorsing Sheehy, praising him as "an American Hero and highly successful Businessman." Trump also expressed respect for Rosendale, stating he would endorse him again if he decided to change course and run for his congressional seat. However, in this instance, Trump believes Sheehy is the candidate best positioned to defeat Jon Tester and regain the Republican majority in the Senate.

Both Sheehy and Rosendale have endorsed Trump's presidential run, adding an intriguing dynamic to the primary race.

The Montana contest will serve as a test for national GOP leaders who have taken a more active role in primaries this cycle, aiming to avoid the nomination of flawed candidates that have cost them seats in the past.

Senator Steve Daines criticized Rosendale for entering the primary, expressing concerns about the party's ability to flip the seat in November. "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," Daines stated, reaffirming his support for Sheehy.

Rosendale, who was first elected to the US House in 2020, has occasionally clashed with GOP leadership, voting to oust Kevin McCarthy as speaker and initially opposing McCarthy's speakership bid in January 2023. House Speaker Mike Johnson, meanwhile, announced that he will not be endorsing Rosendale after reports suggested otherwise.

Senator Jon Tester, one of the most vulnerable Democrats up for reelection in 2024, is running for a fourth term in the red state of Montana. His campaign has been active in airing positive advertisements, highlighting his farming background, support for public lands, Second Amendment rights, and his dedication to defending Montanans' way of life. Tester's most recent filings show he ended December with more than $11 million in cash on hand. Sheehy, on the other hand, had over $1.2 million in the bank at the end of the year, while Rosendale had nearly $1.7 million available in his House campaign account as of December 31, which can be transferred for use in his Senate bid.

Rosendale, who previously served in the Montana Legislature and as state auditor, moved to Montana in 2002. His Maryland roots have been a frequent target of Democratic attacks during his 2018 Senate race against Tester, where he narrowly lost by less than 4 points.

The upcoming primary in Montana will undoubtedly have far-reaching implications for both the Republican Party and the closely watched battle for control of the United States Senate in 2024.