Representative Matt Rosendale, a Montana Republican, on Friday entered the state’s race for Senate, setting off a potentially divisive primary race in a crucial national battleground for control of the chamber.
Mr. Rosendale enters the race from the far-right corner of the party. He is a staunch opponent of abortion rights who voted to overturn the 2020 election, and he played a key role last year in ousting Representative Kevin McCarthy, a fellow Republican, as House speaker.
But while that résumé would normally make him a darling of Donald J. Trump’s political movement, many of the former president’s loyalists have aligned behind Tim Sheehy, a retired Navy SEAL and founder of an aerial firefighting company who began his own Senate campaign in July.
The winner of the primary will face Senator Jon Tester, a Democrat seeking his fourth term. Mr. Tester has been one of the nation’s most popular senators, according to polling from Morning Consult, but is viewed as a vulnerable incumbent because of the deep-red nature of a state that Mr. Trump won by 16 percentage points in 2020. Montana also has a Republican governor and a Republican supermajority in the Legislature.
Before this year, the only time Mr. Tester had shared a ballot with a presidential race was in 2012, when President Barack Obama coasted to a second term. Mr. Obama lost Montana by 13.5 points that year, but Mr. Tester won his race by four points.
In Mr. Tester’s 2018 re-election, he defeated Mr. Rosendale, 50.3 percent to 46.8 percent. That loss factored into the decision from Republican leaders, including Senator Steve Daines of Montana, to recruit Mr. Sheehy into this year’s race. Mr. Daines oversees the party’s Senate races as chairman of the National Republican Senatorial Committee.
Mr. Daines has, in several ways, helped Mr. Sheehy win support from Trump loyalists and deep-pocketed Republican donors, two forces inside the party that have regularly worked at cross-purposes in recent years. Mr. Sheehy has been endorsed by Representative Marjorie Taylor Greene of Georgia, and a super PAC backing Mr. Sheehy has collected millions from wealthy Wall Street executives.
Mr. Sheehy has also contributed about $1 million to his own campaign, which spent more than $4 million last year and entered this year with about $1.3 million on hand.
But while Mr. Sheehy is seeking his first elected office, Mr. Rosendale is a well-known figure in Montana Republican politics. The Senate race will be Mr. Rosendale’s eighth political campaign in the past 14 years. In his previous seven contests — four federal races, two state legislative campaigns and one for state auditor — Mr. Rosendale won five and lost two.
Mr. Rosendale has support from key figures in Mr. Trump’s orbit, including Steve Bannon, the former Trump White House strategist, and Representative Matt Gaetz of Florida. But he is notably missing support from Mr. Trump himself and his son Donald Trump Jr., both of whom repeatedly campaigned with Mr. Rosendale during the 2018 Senate race.
Mr. Rosendale’s reluctance to endorse Mr. Trump’s 2024 presidential bid immediately helped persuade many Trump loyalists to align behind Mr. Sheehy last year, and is a key reason the former president has not yet endorsed a candidate in the Senate race, according to people familiar with the deliberations. Mr. Rosendale did endorse Mr. Trump’s candidacy in December.
Those pro-Trump forces showed their strength on Wednesday when the House speaker, Mike Johnson, retreated from his plan to endorse Mr. Rosendale after facing criticism from top Republican officials and prominent Trump supporters.