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."