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“I didn’t complain about it. I don’t complain. You know what? I’m a big guy. I can take it,” Mr. Santorum said.

Only Michigan Republicans may vote in Tuesday’s GOP primary, though party rules allow voters to change their affiliation temporarily on the spot. The potential involvement of Democrats adds a new twist to a contest already expected to have significant implications for Mr. Romney’s White House bid.

Mr. Romney, who spoke Tuesday with Fox News Channel, called the phone calls “outrageous and disgusting — a terrible, dirty trick.” He accused Mr. Santorum of “teaming up with Barack Obama’s people” to derail the Romney campaign.

“This is a new low for his campaign, and that’s saying something,” Mr. Romney said.

Neither former House Speaker Newt Gingrich nor Texas Rep. Ron Paul has competed actively in Michigan or Arizona.

Mr. Santorum’s rise in polls following the three-state sweep has forced Mr. Romney to work hard in Michigan over the past week. He’s hosted nearly a dozen public events, and he and his allies have spent more than $2 million on local television advertising.

Mr. Santorum campaigned Tuesday around Grand Rapids, in a western Michigan region that is home to many social conservatives and tea party supporters. He also darted to Ohio, one of 10 states voting on next Tuesday — Super Tuesday — for a rally.

“I don’t trust him,” Carol Alexander of Wyoming, Mich., said of Mr. Romney as she waited for Mr. Santorum to arrive at the Rainbow Grille in Grandville, Mich.

A self-described religious conservative, Ms. Alexander said she was leaning toward Mr. Santorum, who she said “speaks what he believes.”

Ms. Alexander said that she’s been inundated with phone calls from campaigns and that “it’s been getting kind of nasty.” But she discounted the impact of Mr. Santorum’s latest tactic.

“Do you really think a liberal is going to vote for Santorum?” she asked with a smile. “I don’t think they’re going to do it.”

Mr. Romney spent part of the day in suburban Detroit, an area with a larger collection of the moderate Republicans who are a key segment of his support.

His overwhelming advantages in Michigan, however, may not pay off in a contest generally dominated by the Republican Party’s more conservative flank. He trailed Mr. Santorum by a significant margin in polls as recently as last week, but those surveys since have tightened and turned Tuesday’s election into a tossup.

Mr. Romney predicted victory Monday night as a crowd at the Royal Oak Music Theatre waited to hear rocker-rapper Kid Rock perform.

“I’m going to win in Michigan, and I’m going to win across the country,” Mr. Romney said.

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