Republican candidate Richard Mourdock is betting Mitt Romney will be the difference in his tough Senate race in Indiana. 

In a new ad for Mourdock, Romney backs the Indiana state treasurer and touts his efforts to balance the state budget and make government more accountable. 

"This fall I'm supporting Richard Mourdock for Senate," Romney says in the ad. "As state treasurer, Richard worked with Gov. Daniels to balance the budget and make government more accountable. As senator, Richard will be the 51st vote to repeal and replace government-run healthcare. Richard will help stop the liberal Reid-Pelosi agenda. There's so much at stake. I hope you'll join me in supporting Richard Mourdock for U.S. Senate."

Mourdock is locked in a tight race with Rep. Joe DonnellyJoseph (Joe) Simon DonnellyDems seek to leverage ObamaCare fight for midterms Todd Young in talks about chairing Senate GOP campaign arm The Hill's Morning Report — Sponsored by PhRMA — Primary results give both parties hopes for November MORE (D-Ind.) after defeating longtime Indiana Sen. Dick Lugar (R) in a primary. 

The bitter primary fight left Republicans bruised, and Mourdock has had trouble uniting Indianans behind his candidacy. Mourdock is running well behind Romney in polls in the Republican-leaning state and has struggled to win over centrist Republicans and independents who backed Lugar. 

The ad is unusual for Romney, who hasn't been used much in advertising for Republican candidates. But Mourdock has been tied with Donnelly in most polls of the race, largely because many Republicans and conservative-leaning independents remain undecided. Romney's stamp of approval could help bring Republicans back into the fold for Mourdock in the tight race.

Watch the ad: