President Barack Obama claimed a "victory" for all Americans after the Supreme Court upheld his reforms to extend health insurance to another 32 million US citizens.
Obama said the ruling, which prescribes how health care will be delivered in the United States for decades to come, went beyond politics as he urged a divided America to get behind a law that was in the interests of all.
"Whatever the politics, today's decision was a victory for people all over this country whose lives are more secure because of this law and the Supreme Court's decision to uphold it," Obama said.
"I'm as confident as ever that when we look back five years from now or 10 years from now, or 20 years from now, we will be better off because we had the courage to pass this law, and keep moving forward."
But Republicans, choking at the very thought of endorsing Obama's signature domestic achievement just five months out from a tightly-contested presidential election, vowed to tear down the law.
"'Obamacare' was bad policy yesterday. It's bad policy today," said Republican presidential challenger Mitt Romney, vowing to repeal on day one a law he has painted as government overreach, costly and wrong-headed.
But Obama said an America struggling to emerge from a painful recession, with a ballooning budget deficit and stubbornly high unemployment, could ill afford to refight old political battles.
"The highest court in the land has now spoken," he said. "With today's announcement, it is time for us to move forward to implement and where necessary improve on this law."
The key provision that underpinned the overhaul, an "individual mandate" requiring almost every US citizen from 2014 to take out health insurance or be subject to a fine, was upheld by a nail-biting 5-4 vote.
John Roberts, the conservative-leaning chief justice often viewed as a bete noire by Democrats, was the unlikely hero for liberals as the key swing vote.
Writing the majority opinion in a landmark ruling that could alter the perception of the Supreme Court, Roberts said the law was constitutional as the punishment for not paying the mandate could be seen as a tax.
"The federal government does not have the power to order people to buy health insurance," Roberts wrote. "The federal government does have the power to impose a tax on those without health insurance."
Justice Anthony Kennedy wrote a stinging dissent on behalf of the four other conservative-leaning justices. "In our view, the entire Act before us is invalid in its entirety," it said.
Despite the worst fears of the Obama administration, the only restriction to the sweeping reforms was on the provision to expand coverage to 16 million more poor Americans through the federal Medicaid program.
The court upheld the clause but ordered the government to withdraw its threat to withhold federal funding from states that do not comply.
The ruling offers new ammunition to Republicans trying to fire up the conservative base and get out the vote in November, but it was also a huge win for Obama that boosts his claim to be a transformative president.
"It is as big an act as killing Osama bin Laden in terms of giving his presidency a temporary boost," Stephen Ryan, former general counsel of the Senate Committee on Governmental Affairs, told AFP.
Two years after Obama signed into law the act to insure most of the 50 million uninsured Americans and prevent coverage from being refused on the basis of patients' medical histories, its fate lay in the hands of just six men and three women.
There were cheers outside the court and across the country from liberals and Democrats, for whom this issue has been a burning crusade for decades.
"We are jumping for joy," said Julie Walters, a 35-year-old recruiter in Novato, California whose three-year-old Violet has a rare genetic condition that causes severe epilepsy.
"It means we can live our life not in fear," Walters, whose daughter could have lost coverage due to her pre-existing condition if the law had been struck down, told AFP.
Secretary of State Hillary Clinton, an integral part of her husband Bill's unsuccessful effort as president to pass health care reform in the 1990s, told journalists in Saint Petersburg she was "excited" at the news.
But "Obamacare" opponents were incensed.
"The entire law must be repealed," said Matt Smith of Catholic Advocate, a group outraged by the law's requirement for organizations to include contraception in their employee health plans.
In another sign of the bitter political climate, Obama's top justice official, Attorney General Eric Holder, was held in contempt Thursday by the House of Representatives for withholding documents related to a botched gun-running operation.
The White House dismissed the move -- the first time in history such a censure had happened -- as a "transparently political stunt."
Although the United States is the world's richest nation, it is the only industrialized democracy that does not provide health care coverage to all its citizens.