Polls show a dead heat between Romney and Santorum, who is playing up his family's blue-collar background as the grandson of a Pennsylvania coal-miner. "This race is close. This race is winnable. But you've got to want it," Santorum told tea party members in St. Clair Shores.
Romney's attacks are a potential problem for Santorum because he's based his candidacy on presenting himself as an uncompromising conservative, contrasting himself with Romney. The former Massachusetts governor has struggled at times to explain why he's changed his position on abortion and other issues.
Santorum compared the health care bill Romney signed in Massachusetts in 2006 with the one Obama signed in 2010. The federal program is wildly unpopular with conservatives.
"Are you going to vote for someone that says one thing one day anything else the next day that's necessary to win? Or are you going to vote for someone you trust?" Santorum asked the crowd in Troy.
Santorum later made a quick detour to Tennessee, a Super Tuesday state that's gotten much less attention, to speak at a tea party rally in a large church in Chattanooga.
In Tennessee, Santorum rebuked Romney for backing the Wall Street bailout. He acknowledged that he and Romney opposed the auto industry bailout, and said Romney was inconsistent.
"I didn't pick and choose based on who my friends are," Santorum said to loud applause. "These are the biggest issues of this race. And we need a candidate who isn't compromised on every single one of them."
Santorum called Obama "a snob" for saying every American child should be able to go to college.
"Why does Obama want everybody to go to college? So his liberal college professors can be indoctrinating people like he has," Santorum said, drawing a long ovation.
Associated Press writer Brian Bakst in Burlingame, Calif., Charles Babington in Chattanooga, Tenn., and Tim Martin in Mount Pleasant, Mich., contributed to this report.
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