WASHINGTON: President Barack Obama's overhaul of the failing US health care system could either be struck down, upheld, or a bit of both in a Supreme Court ruling Thursday with major implications for his re-election bid and the future of the United States.

Two years after Obama signed into law an act to insure another 32 million Americans and prevent coverage from being refused on the basis of patients’ medical histories, its fate lies in the hands of six men and three women.

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.

Underlining the size and scope of the decision, the nine Supreme Court justices held almost six hours of oral arguments over three days in late March, the longest time allotted to a single issue in more than 45 years.

“More people have paid attention to this case than any other case in recent memory, probably with the exception of Bush v Gore,” Paul Clement, who argued the case on behalf of the law's challengers, told reporters last week.

Obama issued a staunch defense of his signature domestic policy achievement at a fundraiser in Miami on Tuesday night.

“I believe health reform was the right thing to do,” he said. “I believe it was right to make sure that over three million young people can stay on their parent's health insurance plan.

“I believe it was right to provide more discounts for seniors on their prescription drugs. I believe it was right to make sure that everybody in this country gets decent health care and is not bankrupt when they get sick.

“That's what I believe. But it's up to you. You decide.” Republicans, who have vowed to repeal “Obamacare,” say it will increase costs, cause insurance premiums to rise and hurt the quality of health care.

“Whatever the Supreme Court does tomorrow, one thing we know: If I'm elected president, we're going to get rid of Obamacare and replace it with real reform,” Romney said at a campaign event in Virginia on Wednesday.

At the heart of the Patient Protection and Affordable Care Act, dubbed “Obamacare” by its critics, lies the individual mandate that requires every US citizen from 2014 to take out health insurance or be subject to a fine.

Opponents argue that Congress overstepped its constitutional prerogatives in requiring individuals to buy insurance, while the Obama administration contends that the move was vital and in line with existing trade and tax law.

“If the court broadly declares that the mandate is beyond Congress's commerce and taxing powers, the ruling could have the very significant effect of turning health reform into a state and local issue rather than a federal issue,” constitutional lawyer Elizabeth Papez told AFP.

“If the court invalidates the mandate on narrower legal grounds, it might simply mean that Congress has to try to write a more careful law.”

There is also the question of the law's expansion of the Medicaid health program for low-income families; a move the CBO says will cover an estimated 16 million uninsured, poor Americans.

Opponents argue this expansion is unconstitutional as it forces states to provide coverage to this new group with an implicit threat to withhold federal Medicaid funds.

The reforms were supposed to be the president's biggest domestic triumph, so he would obviously take a knock if the Supreme Court strikes them down, and critics would be able to paint him as a far weaker figure.

As governor of Massachusetts, Romney signed into law health reforms including an individual mandate that became a template for “Obamacare,” leaving him open to accusations of hypocrisy on the issue.