Representative Matt Rosendale of Montana, the right-wing candidate vying for the state’s contested Senate seat this year, is dropping out of the Republican primary race, he said in a statement on Thursday.

He announced his withdrawal less than a week after he formally entered the race. Just hours after he began his campaign, former President Donald J. Trump endorsed Tim Sheehy, a businessman and retired Navy SEAL who was Mr. Rosendale’s opponent in the primary. With the party establishment seeming to line up behind Mr. Sheehy and Mr. Trump also giving his support, Mr. Rosendale struggled to see a path forward.

“By my calculations with Trump endorsing my opponent and the lack of resources, the hill was just too steep,” Mr. Rosendale said in a statement, adding that he had spoken with Steve Daines, the state’s Republican senator, who is also in charge of Senate Republicans’ re-election efforts. “We both agree that this is the best path forward for Republicans to regain the majority in the U.S. Senate.”

The news of Mr. Rosendale’s withdrawal was earlier reported by Politico. Mr. Rosendale did not say whether he would endorse Mr. Sheehy. The news sets up Mr. Sheehy as a favorite to secure the nomination and face Jon Tester, the Democratic incumbent, in November.