G.O.P. Primary Fight for Senate Begins in Montana, a Top Battleground

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