Former President Donald J. Trump endorsed Tim Sheehy for the Republican Senate nomination in Montana on Friday, putting his powerful imprimatur on the primary race the same day that one of his longtime allies, Representative Matt Rosendale, announced his campaign.

Mr. Trump and Mr. Rosendale had campaigned together in 2018, when Mr. Rosendale’s first attempt to unseat Senator Jon Tester, a Montana Democrat, failed. But the House lawmaker’s loss in that race — and his failure to endorse Mr. Trump’s latest presidential bid sooner — convinced the former presidential to endorse Mr. Sheehy.

In a statement, Mr. Trump said he respected Mr. Rosendale and would be happy to endorse a House bid if he would change his mind about running for the Senate. He also noted Mr. Sheehy’s support from Senator Steve Daines, a Montana Republican overseeing the Republican Senate campaign arm, who has developed a close relationship with the former president.

“Tim is the candidate who is currently best positioned” to defeat Mr. Tester, Mr. Trump said.

Mr. Rosendale entered the race on Friday 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 Mr. Trump’s political movement, many of the former president’s loyalists have aligned behind Mr. Sheehy, a retired Navy SEAL and founder of an aerial firefighting company who began his own Senate campaign in July.

A telegenic father of four young children and recipient of a Bronze Star and a Purple Heart Medal, Mr. Sheehy meets the “central casting” look that Mr. Trump seeks in many candidates.