Former Massachusetts governor Mitt Romney, who is seen as a possible front-runner for the 2012 Republican presidential nomination, on Thursday will confront the issue that poses his greatest political risk: health care.

Romney’s office is billing his speech at the University of Michigan Cardiovascular Center in Ann Arbor as the unveiling of “a plan to repeal and replace Obama­care.”

But that is tricky territory for Romney, given the fact that the new national health-care law, enacted last year, is similar in design and ambition to the greatest achievement of his tenure as governor — a landmark 2006 law that established Massachusetts as the first state in the nation to guarantee health insurance coverage for all its residents.

President Obama has cited the Massachusetts law as a model for the one that he and the then-Democratic Congress put into place with virtually no Republican support.

Among the features shared by the two laws is a requirement that adults who are not covered by their employers or by a government program must purchase health insurance or face a financial penalty.

This “individual mandate” is the most controversial provision of the federal law, and its constitutionality is under a legal challenge that appears headed for the U.S. Supreme Court.

Romney, who has formed a presidential exploratory committee, has defended that provision in the Massachusetts law as crucial to ensuring universal coverage and to making sure that those who can afford insurance are not taking unfair advantage of the system.

“Some of my libertarian friends balk at what looks like an individual mandate,” Romney wrote in the Wall Street Journal as he was preparing to sign the law in April 2006. “But remember, someone has to pay for the health care that must, by law, be provided: either the individual pays or the taxpayers pay. A free ride on government is not libertarian.”

However, he has also said that what works in one state is not necessarily a good idea in others.

Romney’s rivals, however, are unlikely to accept that as much of a distinction. “Looking at the Massachusetts experience, it would not be one I would want for the country to follow any further,” former Minnesota governor Tim Pawlenty told the Nashua Telegraph in New Hampshire last year.

GOP and ‘Romneycare’

As the primary campaign kicks into higher gear, Romney is feeling greater pressure to inoculate his record and to spell out how he would approach the issue differently from 1600 Pennsylvania Ave.

Some conservatives view “Romneycare” as such an apostasy that they say nothing short of an act of contrition — similar to the one Pawlenty has offered for his previous support for a cap-and-trade system to deal with climate change — will do.

Romney is “going to be clubbed over the head on health care for the duration of the primaries, with Thursday’s speech sure to provide plenty of new material to club him with,” wrote Allahpundit, a blogger on the influential conservative Web site Hot Air. “Pawlenty’s already been working to set up a contrast with him by apologizing for his own prior support for cap and trade. Anyone think this speech, lacking any mea culpa, will blunt the force of those coming attacks?”

In choosing a venue for such a potentially pivotal speech, Romney opted not to go to Massachusetts but rather to Michigan — an economically depressed state where he grew up and where his father, a former auto executive, served as governor. It is also where Romney announced his 2008 campaign for president.

Romney’s office said his approach to revamping the national health-care system would rely on five broad principles: restoring to the states the responsibility to care for the poor, uninsured and chronically ill; providing a tax deduction to individuals who buy health insurance, similar to one that employers can claim for insuring their workers; streamlining federal health-care regulation; reducing malpractice suits; and making the health-care market more efficient.

‘Some things I’d change’

Romney has said there are some aspects of the Massachusetts law that he would have done differently — indeed, some parts of it were passed by the Democratic legislature over his veto.

“Some things worked, some didn’t, and some things I’d change,” the former governor said at a presidential candidates forum in March in New Hampshire. But he added: “One thing I would never do is to usurp the constitutional power of states with a one-size-fits-all federal takeover.”

Aides said that in his speech on Thursday, Romney will not renounce the major features of the Massachusetts law or his role in getting it passed.

The Massachusetts health-care system has encountered some criticism in the five years since it was put into place. Medical costs rose more quickly than projected in its initial years, and there have been some complaints about physician shortages. But it has succeeded in ensuring that nearly the entire population has some kind of health coverage, and polls indicate that it enjoys strong support.

At the time that Romney put together the plan that became the law, however, Massachusetts’s health-care system was already in better shape than those in many other states. The percentage of uninsured among Massachusetts nonelderly was 13.2 percent, compared with a national average of 17.8 percent. And the state had a stronger safety net, thanks to a $1.1 billion state fund to reimburse hospitals and health centers that provided care to those who couldn’t pay for it.

Romney also had a strong incentive to take action: At the time, Massachusetts faced the loss of $385 million a year in Medicaid money. He and the late senator Edward M. Kennedy (D) worked together closely to persuade Tommy Thompson, then health and human services secretary, to let Massachusetts keep the money if it could pass a plan to get all of its residents covered.

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Staff researcher Lucy Shackelford contributed to this report.