It can be difficult to remember now, given the ferocity with which many Republicans assail it as an attack on freedom, but the provision in President Barack Obama’s health care law requiring all Americans to buy health insurance has its roots in conservative thinking.

The concept that people should be required to buy health coverage was fleshed out more than two decades ago by a number of conservative economists, embraced by scholars at conservative research groups, including the Heritage Foundation and the American Enterprise Institute, and championed, for a time, by Republicans in the Senate.

The individual mandate, as it is known, was seen then as a conservative alternative to some of the health care approaches favored by liberals — like creating a national health service or requiring employers to provide health coverage.

"In 1993, in fighting ‘Hillarycare,’ virtually every conservative saw the mandate as a less dangerous future than what Hillary was trying to do," Newt Gingrich, the former speaker of the House, said at a debate in December, casting his past support of a mandate as an antidote to the health care overhaul proposed by Hillary Rodham Clinton during her husband’s administration. Since then the politics of health care have grown more twisted and tangled than the two snakes entwined around the staff in a caduceus, which is sometimes used as a symbol of medicine. It is now Republicans and conservatives who oppose the individual mandate, arguing that it is unconstitutional, while Democrats, who were long resistant to it, are its biggest defenders.

Democratic health care analysts have been taken aback by the speed with which Republicans have made the individual mandate a symbol of socialist totalitarianism to much of their base.

"I noted the irony of a Republican idea being the source of Republican opposition," said Neera Tanden, the president of the Center for American Progress, a liberal research group, who served in the Obama administration and as the policy director for Hillary Clinton’s presidential campaign in 2008. And longtime supporters of the mandate, who for years had believed the biggest obstacle to enacting it was attracting Democratic support, saw Democrats become its last supporters. "It totally flipped," said Peter Harbage, a health care analyst who has advised Democratic and Republican supporters of individual mandates.

These shifting political winds have become a major factor in the presidential campaign. Mitt Romney is often challenged by Republican rivals about the health care law he signed as the governor of Massachusetts, which also contains an individual mandate. Gingrich is often asked about his years of support for the idea. And Obama — who opposed the individual mandate four years ago as a candidate, but came to accept it as president — is now waiting to see whether the Supreme Court upholds the idea or strikes it down.

Some conservatives originally saw the individual mandate as a way to make certain that uninsured people who became ill or were injured — but were still entitled by law to medical treatment — did not push the cost of their care onto others.

"If a young man wrecks his Porsche and has not had the foresight to obtain insurance, we may commiserate, but society feels no obligation to repair his car," Stuart Butler, a distinguished fellow at the Heritage Foundation, said in a 1989 lecture on how to ensure affordable health care for all Americans. "But health care is different. If a man is struck down by a heart attack in the street, Americans will care for him whether or not he has insurance."

Part of Butler’s solution back then? "Mandate all households to obtain adequate insurance."

But Democrats were leery of the idea. In 1993, when President Bill Clinton put forward the broad outlines of his health care proposal in a speech to the National Governors Association in Tulsa, Okla., he called for requiring employers to buy coverage for their workers.

The idea of an individual mandate, he noted in the speech, "has found some favor in the United States Congress, primarily among Republicans, but not exclusively, because it has the appeal of not imposing a business mandate, which has a bad sound to it."

To combat Clinton’s proposal, a large group of Republican senators, including the minority leader at the time, Bob Dole, and several others who are still in office, proposed a bill that would have required individuals, and not employers, to buy insurance.

After Clinton’s health care proposal died, Democrats began searching for more politically palatable solutions.

But there was still little Democratic support for an individual mandate, as Sen. John Breaux of Louisiana found when he proposed one several years later. But officials, centrist policy groups and even liberal research organizations slowly warmed to the idea.

When Sen. John Edwards sought the Democratic presidential nomination in 2004, he proposed an insurance mandate for children, but not adults. The politics of the mandate really began to change when Massachusetts enacted its health care law. That effort united Romney, a Republican, with Sen. Edward M. Kennedy, a liberal stalwart who got involved in part to protect the state’s share of federal Medicaid money, and Democratic lawmakers on Beacon Hill. All wound up supporting a plan with an individual mandate.

"Democrats, based on the Massachusetts experience, became much more comfortable with the idea of an individual mandate," said John McDonough, a professor at the Harvard School of Public Health who was a health care advocate in the state at the time.

When Edwards ran for president again in 2008, his health plan called for an individual mandate for all. Hillary Clinton followed suit. But Obama resisted the idea as a candidate, calling for a mandate for children only.

But as president, Obama changed his mind.

Many conservatives changed their minds too, however. Some of the Republican senators who once supported versions of the individual mandate railed against Obama’s plan. Romney, despite signing a similar plan into law in Massachusetts, has made a pledge to try to "repeal Obamacare" central to his presidential campaign.

Butler, of the Heritage Foundation, said in an interview that his views had evolved over the years. He said that he now believed that near-universal coverage could be achieved without a mandate, and added that the mandate he once supported was for basic catastrophic coverage, not comprehensive insurance.

The Heritage Foundation has taken pains to distance itself from its past support of an individual mandate: It wrote a court amicus brief noting its change of heart, and Butler wrote an op-ed in USA Today this month headlined "Don’t blame Heritage for ‘ObamaCare’ mandate."

But not all of the early conservative proponents have changed their minds. Mark V. Pauly, a professor of health care management at the University of Pennsylvania’s Wharton School who was a co-author of an influential 1991 paper that called for an individual mandate, said that he was discouraged to see so many Republicans shunning an idea they had once supported.

"My view was, I still agree with myself," he said in an interview. "And I was pleased that this thought was getting bipartisan appeal where the whole thing was declared dead on arrival by Democrats 18 years before."

© 2012 The New York Times Company