(Image Credit: Chip Somodevilla/Getty Images)

BOISE, Idaho - For the first time since Rick Santorum has surged in the polls, Mitt Romney went on the offense, warning voters that if they want to elect a fiscal conservative they "can't vote for Rick Santorum."

"There are a bunch of Republicans running, well, we're down to four now," said Romney, speaking at Guerdon Enterprises, a manufacturer of prefabricated homes. "But there were a bunch, now we're four. And of those four we'll all hear people talk about what they'll do to try and shrink the size of the federal government to cut back on spending."

"But you have to look at people's records as well as their words," said Romney. "And I know that Senator Santorum is getting his moment in the spotlight now, which is a good thing."

"I hope people take a very close look at his record," he warned. "Because he was in Congress for about 20 years and during that time the size of the federal government doubled during his time in office."

While Romney's campaign has held daily conference calls with the press to discuss Santorum's record, Romney himself has only made veiled references to the former Pennsylvania senator.

But today in Idaho, just after news broke that a former backer of his - Ohio Attorney General Mike DeWine - had left his camp to support Santorum, Romney was fired up speaking about his rival.

"And by the way he voted to raise the debt ceiling five different times without compensating cuts," said Romney. "And he's a big proponent of earmarks. He voted for billions of dollars of earmarks including the bridge to nowhere."

"If you want a fiscal conservative you can't vote for Rick Santorum 'cause he's not, he's not a deficit hawk, he says he's not a deficit hawk," said Romney. "I am. I'm a fiscal conservative. I'll balance the budget."

Reporters shouted questions at Romney about DeWine's decision to back Santorum but Romney did not respond and walked away.