MARQUETTE, Mich. -- The question of whether Mitt Romney is conservative enough to deserve the Republican presidential nomination regained center stage in the GOP contest Sunday, with Rick Santorum saying the former Massachusetts governor fails the test.

Santorum urged Michigan voters to turn the race "on its ear" by rejecting Romney in Tuesday's primary in his native state, in which Romney is spending heavily to avoid an upset. Santorum said Romney's record is virtually identical to President Barack Obama's on some key issues, especially mandated health coverage, making him a weak potential nominee.

"Why would we give away the most salient issue in this election?" an impassioned Santorum told more than 100 people in a remote, snow-covered region of Michigan's Upper Peninsula. Santorum, a former senator from Pennsylvania, said he is the true conservative on fiscal and social issues.

Romney rejected the claims.

"The biggest misconception would be that I'm a guy that comes from Massachusetts and therefore I can't be conservative," Romney told "Fox News Sunday."

In his one term as Massachusetts governor, he said, he balanced budgets, reduced taxes, enforced immigration laws, "stood up for traditional marriage" and was "a pro-life governor."

"I'm a solid conservative," Romney said.

The exchange highlighted the choice facing Republican voters in Arizona and Michigan on Tuesday, and another 10 states a week after that.

Romney did pick up the endorsement of Arizona Gov. Jan Brewer on Sunday.

Conservative activists dominate the GOP primaries. But party regulars fear too much focus on the Republican right will leave the eventual nominee poorly positioned to confront Obama in November, when independent voters will be crucial.

Santorum, a hero to anti-abortion and home-schooling advocates, disputes that argument. The way to beat Obama, he said Sunday, is with an unvarnished conservative whose views dramatically clash with the president's on the economy, church and state, energy, foreign policy and other issues. He said the party needs "someone who can paint a very different vision of the country."

Romney and Santorum hit Obama on many issues, including the president's apology for the actions of U.S. troops who burned Qurans -- inadvertently, they said -- while destroying documents on a military base in Afghanistan.

Romney said that for many Americans, the apology "sticks in their throat."

"We've made an enormous contribution to help the people there achieve freedom," he said. "And for us to be apologizing at a time like this is something which is very difficult for the American people to countenance."

As for U.S. troops in Afghanistan, Romney said Obama "made an enormous error by announcing the withdrawal date of our surge forces during the fighting season."