Former Massachusetts Gov. Mitt RomneyWillard (Mitt) Mitt RomneyWhat sort of senator will Mitt Romney be? Not a backbencher, even day one Trump endorses Romney in Utah Senate race Obama failed on Russia; Trump must get it right MORE (R) is endorsing Rep. Mike SimpsonMIchael (Mike) Keith SimpsonHouse retirement sets off scramble for coveted chairmanship The Hill's 12:30 Report Even some conservatives seem open to return to earmarks MORE (Idaho), an establishment Republican facing a tough Tea Party challenge.  

"I'm proud to endorse Mike Simpson for Congress," the former Republican presidential nominee says in a statement released by Simpson's campaign. "Mike is a proven conservative who's fought to repeal ObamaCare and control government spending."

Simpson is a close friend and ally of Speaker John Boehner (R-Ohio), and has carved a more centrist voting record while in Congress. He's fighting off a challenge from attorney Bryan Smith (R), who has the backing of the Club for Growth, a deep-pocketed conservative group.

Romney's endorsement wouldn't usually help a candidate fend off a Tea Party challenge — he's no hero to hardline conservatives. But Idaho is heavily Mormon and Romney dominated both the primary and general election in the state.  

"I'm truly honored to have Gov. Romney’s endorsement," Simpson said in response. "He is a tremendous example of leadership and integrity. I was proud to be an early backer of his campaign for president and his support means a great deal to me personally."