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“You can’t win Michigan or Pennsylvania unless we are able to get conservatives who don’t necessary vote with us all the time to join us in the race,” he said.

Exit polling conducted by the Associated Press and broadcast networks found that 9 percent of voters were self-identified Democrats, which was more than in 2008, but less than in the 2000 primary here.

Mr. Santorum won 53 percent of those Democrats, while Mr. Romney led among Republicans, and the two were tied among independent voters.

In Arizona, Mr. Romney won in nearly every demographic group.

But in one worrisome sign for him, exit polling showed he won just 33 percent of the Hispanic vote in Arizona — well behind his share of the vote there overall. Mr. Santorum, meanwhile, picked up 31 percent.

Voters backing Mr. Santorum said he appealed to them as a conservative they could relate to.

“He’s conservative and seems to have his feet planted on the ground well. I don’t get that sense from Romney when I hear him speak,” said Karen Lanning, a 64-year-old retiree from Jenison, Mich. “When I hear him speak, I don’t know that he really has the heart for the common man. It is a total disconnect, whereas Santorum, you feel that connection.”

Mr. Romney, meanwhile, was winning over voters who said their chief concern was selecting a candidate who could beat Mr. Obama in November.

That view won him the support of Arizona Gov. Jan Brewer over the weekend, and of voters like Bob Merrill, a 27-year-old insurance agent from Portage, Mich., who said the election was about “electability.”

“There are a lot of young voters that are one-issue voters. They will vote against a guy that campaigns against abortion,” Mr. Merrill said. “Someone like Rick Santorum chooses to separate himself from Mitt Romney on socially conservative issues, which is great in a primary when you are trying to out-Republican the other person, but when you go into the general election and you need those moderate voters, it is not going to help at all.”

Stephen Dinan reported from Tempe, Ariz.