BOISE, Idaho -- Drawing yet another line in his battle for the Republican nomination, Mitt Romney urged voters here to take a closer look at Rick Santorum's Senate record, which he said would disqualify former Pennsylvania senator as not a true fiscal conservative.

"You have to look at people’s records as well as their words. 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. 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," Romney said. "And by the way, he voted to raise the debt ceiling five different times without compensating cuts. And he’s a big proponent of earmarks. He voted for billions of dollars of earmarks including the bridge to nowhere" in Alaska.

"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. I am. I'm a fiscal conservative. I’ll balance the budget," Romney said.

Romney's assault on Santorum on Friday at a large and energetic rally outside Boise was the first time he has mentioned Santorum on the stump since the day after the former senator swept contests in Colorado, Minnesota and Missouri to emerge as the latest challenger to Romney's frontrunner status. A Detroit News poll yesterday showed Santorum leading Romney in Michigan, the former Massachusetts governor's home state, by four points, and the Romney campaign has engaged accordingly.

In recent days campaigning in Michigan and Ohio, Romney held his fire on Santorum, focusing on attacking "union bosses" and defending his stance on the 2009 auto bailouts. Meanwhile, Romney surrogates engaged in a media campaign against the former senator, holding conference calls with reporters each of the last three days to label Santorum in a variety of ways, including a "pundit" and a "nice guy" who simply "couldn't say no" to Washington spending. 

Romney's campaign and the super PAC backing him, Restore our Future, have also more than doubled the ad spending of Santorum and his allies in Michigan, spending $3.9 million there to Santorum's $1.5 million, according to NBC ad tracking sources.

On Friday, Santorum scored a victory against Romney in Ohio just before Romney's event in Boise was set to begin. Ohio Attorney General Mike DeWine, a delegate in Ohio, switched his endorsement from Romney to Santorum, declaring "To be elected president, you have to do more than tear down your opponents."

Romney's campaign, which has racked up endorsements from local and national figures across the country, waved off the defection, in part by downplaying the importance of endorsements in the first place. Former New Hampshire Gov. John Sununu told reporters on a conference call, "The most important thing you can get from an endorser is their organization. And frankly, as I understand it, attorney generals don't have that much of a political organization,"

Senior Romney adviser Eric Fehrnstrom noted that DeWine had supported former Minnesota Gov. Tim Pawlenty before switching support to Romney when Pawlenty ended his candidacy. Fehrnstrom looked to shrug off the switch, tweeting: "Mike DeWine makes his third endorsement in six months -- Pawlenty, Romney, and now Santorum. I think that's a record."

Asked by NBC after the Boise event for comment on DeWine's change of heart, Romney did not respond, and aides led him to his car, saying he had a flight to catch.