Mitt Romney appears in a new ad backing Rep. Mike Simpson (R-Idaho) against his a tea party challenger, marking the first time Romney has vouched for a congressional candidate on the airwaves in 2014.

"You can take it from me," Romney says in the ad from the U.S. Chamber of Commerce. "The conservative choice for Congress is Mike Simpson. I know because I've seen him in action." Romney goes on to tout Simpson's record on cutting spending and fighting Obaamcare.

Romney endorsed Simpson last year, but the commercial marks his most direct engagement yet in the race. Idaho has a large Mormon population, which means Romney's endorsement could mean more than it does in most districts. Simpson and Romney are both Mormon.

Simpson is trying to fend off a challenge from attorney Bryan Smith, who has the backing of the anti-tax Club for Growth. The Club went up with a new ad this week attacking Simpson for voting for TARP in 2008.

The Chamber did not say specifically how much money it is spending on the Romney ad. A spokesperson would only say the buy is "sizeable."