Former GOP presidential nominee Mitt Romney endorsed Rep. Mike Simpson, R-Idaho, for re-election in the 2nd District on Monday, touting the congressman as a “true conservative” in a fundraising email.

Simpson, an eight-term Republican, is facing a primary challenge from

from tea-party-backed attorney Bryan Smith, who is receiving support from groups

who is receiving support from groups such as the Club for Growth and FreedomWorks.

The primary will feature one of the marquee battles between the tea party and business in the fight for control of the GOP in the midterm cycle.

“Congressman Mike Simpson is a proven conservative and ‘Idaho tough,'” Romney wrote in the fundraising email. “But some groups from outside Idaho have decided to attack Mike and distort his conservative record. He needs our help to fight back and set the record straight.”

Romney pointed to Simpson’s 40 votes to repeal Obamacare, his co-sponsorship of the balanced budget amendment and Simpson’s A+ record with the National Rifle Association as proof of his conservative chops.

Romney carried Idaho’s 2nd District with 64 percent in 2012. Todd Cranney, Romney’s national political director for the 2012 campaign, is working on Simpson’s re-election campaign.

“I’m truly honored to have Governor Romney’s endorsement,” Simpson said in a release. “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.”