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The Romney camp counters that their boss always has favored a state’s rights approach and that Mr. Santorum’s attack shows desperation as the ex-governor rattles off victory after victory, extending his lead in the delegate count.

In the debates, Mr. Romney has said Massachusetts was a special circumstance and has denied believing that a national mandate is the correct policy. He draws applause from supporters on the campaign trail when he vows to repeal Mr. Obama’s law.

But it is clear that other voters are still wrestling with the topic, as evidenced by a woman who asked him at town hall in Youngstown, Ohio, this week to clarify for her how his plan fits under the conservative umbrella and the president’s does not.

Charles R. Siphan, chairman of the political science department at the University of Michigan, said the Romney defense clearly has resonated with some conservatives, but not all of them.

“It’s a hard argument for Romney to make, and it’s not clear that it has ended up being convincing or that it has helped him,” Mr. Siphan said, before explaining that polls show the public — especially conservatives — have little appetite for an individual mandate.

“Frankly, I’m surprised that Romney’s opponents haven’t brought this issue up more often and more forcefully, as it seems to be a clear winner for them,” he said.

Ohio Attorney General Mike DeWine said the health care system Mr. Romney embraced in the Bay State is a primary reason he withdrew his support from him last month and put it behind Mr. Santorum.

The ex-governor’s record, Mr. DeWine warned, will act as “ball and chain around his neck” in a general election by diminishing the amount of grass-roots excitement and energy behind his candidacy.

“For us to win, we have to have a lot of the people I am seeing coming to the Santorum rallies,” he said. “These are pro-lifers, these are tea partyers, these are home-schoolers — these are the people I saw in the last successful Bush campaign when we went to all the call centers and you had home schoolers with five or six kids making phone calls. You are not going to see that for Romney. That isn’t going to happen. I will be for him if he is the nominee, but you are not going to see that energy.”

Some of the Santorum supporters gathered here echoed a similar sentiment, adding that Mr. Romney has yet to earn their complete trust.

“I don’t know what he would do in the future because of his past,” said Amanda Batalune, 31, a pharmacist from Steubenville. “Like Dr. Phil says, ‘Your past behavior is a predictor of your future behavior.’ I believe that with him, so I don’t know. He could have changed his mind, but I just don’t trust him the way I trust Rick Santorum.”

Donald and Kelly Goldsborough, husband and wife from Delaware, said they drove 497 miles to attend Mr. Santorum’s victory party — carrying a massive wooden sign that filled the bed of their full-size truck that reads: “Vote Santorum To Repeal Obamacare.”

“I almost feel like I’m not sure I would sit home or not, and I have never thought of throwing my vote away,” Mr. Goldsborough said. “I suppose when push comes to shove, I would vote for Romney, but I am just not enthusiastic about him.”