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The Republican message machine is trying hard to accuse President Obama of increasing taxes on middle-class Americans after the Supreme Court’s ruling that the health care mandate is valid as an exercise of the government’s taxing power.

But the party is doing so without the help of Mitt Romney, whose own health plan in Massachusetts contained an almost identical mandate.

On Monday, Eric Fehrnstrom, Mr. Romney’s senior adviser, strayed wildly from the coordinated comments of the Republican leadership on Capitol Hill and other party strategists by saying Mr. Romney, the presumptive Republican presidential nominee, agrees with Democrats that Mr. Obama’s health care mandate is not a tax.

“The governor disagreed with the ruling of the court,” Mr. Fehrnstrom said on MSNBC’s “The Daily Rundown” program. “He agreed with the dissent written by Justice Scalia, which very clearly stated that the mandate was not a tax.”

Chuck Todd, the show’s host, suggested that Mr. Romney “believes that you should not call the tax penalty a tax. You should call it a penalty or a fee or a fine.”

“That’s correct,” Mr. Fehrnstrom said.

The awkwardly off-message response is an example of the difficulty that Mr. Romney has in carrying the conservative mantle on health care — an outcome that Mr. Romney’s Republican rivals predicted during the Republican primaries would come to pass.

Attacks by Mr. Romney on the president’s health care plan inevitably lead to comparisons to his similar plan in Massachusetts. And while Mr. Romney has pledged to completely repeal what conservatives call “Obamacare,” he has been silent on whether its insurance mandate is a tax on Americans.

On Thursday, just hours after the Supreme Court ruling, Mr. Romney’s statement avoided the issue even as lawmakers were taking to the House and Senate floors to gleefully seize on the court’s surprising justification.

The Republican establishment continued that effort over the weekend. On Sunday, Speaker John A. Boehner and other top Republicans insisted that the court revealed a truth that Mr. Obama had wanted to avoid.

“Even though the president tried to admit for, you know, over a year that it wasn’t a tax, nobody believed it, and now we know it,” Mr. Boehner said on CBS’s “Face the Nation” program.

Representative Paul Ryan, Republican of Wisconsin, accused Mr. Obama of purposely misleading Congress in order to get the health care bill passed.

“The president, on your show, said this is not a tax,” Mr. Ryan said on ABC’s “This Week” program. “Then he sent the solicitor general to the Supreme Court to argue that it is a tax in order to get this past the Supreme Court. The broken promises and the hypocrisy are becoming breathtaking.”

On Monday, the National Republican Campaign Committee began running television commercials attacking the health care ruling. In an e-mail to reporters, a committee spokesman accused Democrats of driving “circles in a rhetorical cul-de-sac on the difference between a penalty and a tax.”

Meanwhile, the Republican National Committee tweeted: “White House still desperately trying not to call the healthcare mandate what it is: a tax.”

Democrats fought back against that characterization on Sunday. Representative Nancy Pelosi of California, the minority leader, said on NBC’s “Meet the Press” program that the health care mandate includes a “penalty” for a small number of “free riders” who choose not to buy insurance.

“It’s a penalty that comes under the tax code for the 1 percent, perhaps, of the population who may decide that they’re going to be free riders. But most people are not affected by that,” Ms. Pelosi said. “It’s not a tax on the — it’s a penalty for free riders.”

But as Mr. Romney pursues the presidency for a second time, some of his past statements match up more closely with the Democrats than with the Republicans.

The argument made by Ms. Pelosi on Sunday was the very one that Mr. Romney made in 2006 when he was fighting for his state health care plan. And he repeated that argument in the years after, including during a debate in the 2008 presidential campaign.

“Yeah, we said, ‘Look, if people can afford to buy it, either buy the insurance or pay your own way,’ ” Mr. Romney said during a January 2008 debate in New Hampshire. “Don’t be free riders and pass the cost of health care on to everybody else.”

In an opinion article in USA Today in 2009, Mr. Romney again used the term “free riders,” writing that a penalty like that in his health care plan “encourages ‘free riders’ to take responsibility for themselves rather than pass their medical cost on to others.”

On MSNBC on Monday, Mr. Fehrnstrom tried to stress that Mr. Romney does not believe that the Massachusetts health care plan would work as a federal plan.

“What we did works in Massachusetts,” he said. “It was never meant to be a one-size-fits-all for the entire nation.”

But his comments may have undercut Mr. Romney’s Republican allies, who were hoping the tax argument would be a bit of good news in an otherwise disappointing Supreme Court ruling. Even as Mr. Fehrnstrom was declaring that the mandate is not a tax, the headline under his image said otherwise.

“GOP: Mandate is one big tax,” it read.

Andrea Saul, a spokeswoman for the Romney campaign, urged Mr. Obama to clarify his position on the issue.

“The Supreme Court left President Obama with two choices: the federal individual mandate in Obamacare is either a constitutional tax or an unconstitutional penalty. Governor Romney thinks it is an unconstitutional penalty,” she said. “What is President Obama’s position: is his federal mandate unconstitutional or is it a tax?”