(CNN) - Mitt Romney's wading into a Republican primary battle.
The 2012 GOP presidential nominee Monday endorsed Rep. Mike Simpson of Idaho, who's facing a tough primary challenge as he runs for re-election next year.
"Congressman Mike Simpson is a proven conservative and 'Idaho tough.' 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 says in a statement posted on Simpson's campaign website.
"This stalwart conservative leader shares our values. That's why I am proud to endorse his re-election," Romney adds.
Simpson is facing a primary challenge from Bryan Smith, an attorney who claims that the eight-term congressman is not conservative enough and is soft on spending. Smith is backed by the anti-tax group Club for Growth, which has a history of backing conservative primary challengers.
As he fights back against his primary challenger, Simpson has been touting his seniority and highlighting what he says is a conservative voting record.
Simpson represents Idaho's second Congressional District, which stretches from Boise to the borders with Wyoming and Montana and covers the entire eastern and most of the central parts of the state. Romney's backing could especially help in eastern Idaho, which has a large Mormon population.
Simpson was an early supporter of Romney's 2012 presidential bid.
Simpson is also being backed by House Speaker John Boehner.