PHOENIX (AP) - Mitt Romney took new aim at Rick Santorum's image as a devoted conservative Thursday, accusing his rival of compromising his own principles by repeatedly voting for legislation he didn't believe in.

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Romney noted that during Wednesday night's heated GOP debate the former senator said he had voted for President George W. Bush's No Child Left Behind education bill because "sometimes you take one for the team.""I wonder which team he was taking it for?" Romney asked during a speech to an Association of Builders and Contractors meeting in Phoenix."My team is the American people, not the insiders in Washington."

During the debate, Santorum was forced to defend several votes on issues such as a right-to-work measure that clashed with his conservative philosophy. He explained at length why he backed bills that included targeted spending called "earmarks," saying it was better to have Congress decide where to send the money than to let executive branch departments have complete authority.

"I don't know that I've ever seen a politician explain in so many ways why he voted against his principles," Romney said Thursday.

Romney and Santorum are powering into a crucial stretch of Republican primaries and caucuses over the next 13 days.

Later Thursday, Romney was turning his focus to tea partyers in Michigan, his birthplace, where cash-strapped Santorum is waging an unexpectedly strong challenge. Romney's been put on the defensive in the auto-building state over his opposition to the government's bailout of car makers.

Romney took a pounding on the auto issue in the debate, and President Barack Obama's re-election campaign piled on Thursday.

Obama released a TV ad in Michigan accusing Romney and the other GOP candidates of turning their backs on an industry that supports more than 1 million workers in the state by opposing the bailout. A tea party rally Thursday night in Milford, Mich., will give Romney another chance to explain why he opposed the rescue of GM and Chrysler amid the economic crisis but supported bailouts for banks.

Santorum, flourishing in the polls, trails in the money chase and is concentrating on beefing up his campaign treasury in hopes of an upset in Michigan's primary on Tuesday. That would cap a rebound that began two weeks ago when Santorum won caucuses in Minnesota and Colorado and a nonbinding primary in Missouri.