The imminent US Supreme Court decision on Barack Obama's signature health care reforms has major implications for November's presidential election and for the future of the United States.

A ruling is expected as early as Monday -- two years to the day after Obama signed into law an act to insure an extra 32 million Americans and prevent coverage from being refused on the basis of patients' medical histories.

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 is vital and in line with existing trade and tax law.

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

There are several possible outcomes: the highest court in the land could overturn Obamacare in its entirety, throw out just the individual mandate, endorse the whole law, or reach some other kind of mixed ruling.

It must first decide if the fine for not paying the individual mandate constitutes a tax. If so, it may argue that preemptive challenges are forbidden and decline to rule at all until the "tax" comes into force in 2014.

There is also the question of the law's expansion of the health program for low-income families, Medicaid, which is being portrayed by Republicans as an unconstitutional grab of state funds by the federal government.

While Supreme Court justices are meant to base their decisions on the framework provided by the centuries-old US constitution, many liberals fear the conservative-leaning bench could be swayed by political bias.

"What they're afraid of is that the subjective impulse of the judge, rather than something more objective, will control the decision," Justice Stephen Breyer, viewed as one of the more liberal members of the bench, told AFP.

Article 1, Section 8, Clause 3 of the constitution gives Congress the authority "to regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes."

The Obama administration says this validates the individual mandate, but the state of Florida, and 25 other supporting states, claim it is overreach because it forces people to buy something.

"If they can force you to buy health insurance, they can force you to buy a car, asparagus, a gym membership," Ken Cuccinelli, Republican attorney general for Virginia, told AFP.

Obamacare supporters say health insurance is different because everyone consumes the product and, in a country where nearly one in seven went without medical insurance in 2011, Congress has a duty to protect its citizens.

"If you don't have health insurance, then you show up at the emergency room, doctors are under orders to treat you -- as any Western, any civilized society would do. And who pays for that?" said former Obama administration lawyer Neal Katyal.

"Well, ordinary Americans pay for that. They're the ones who have to pick up the tab for those who don't have insurance. We are not regulating what people buy, we're regulating how people finance it."

Supreme Court justices are expected to take prior case law into account, including related precedents on products as diverse as wheat and marijuana.

"The commerce clause has to apply to a world that changes every five minutes," Breyer told AFP.

"Well, the difficulty of a constitutional judge's task is to work out how... these values that do not change, how do they apply to circumstances that continuously change?"

The court's decision, which must come by Thursday, will have a major impact on health care delivery in the United States for decades to come, but its impact on November's showdown between Obama and Mitt Romney is less certain.

One might expect the Republican challenger to rally the base on the issue, win or lose, but health care raises problems for Romney, too.

As governor of Massachusetts, he signed into law health reforms that included an individual mandate and became a template for "Obamacare."

Trying to guess which way the justices will rule has become a common pastime in Washington.

Some say tough questioning from key swing justice Anthony Kennedy in March indicates all or part of Obamacare will be struck down.

A poll of former Supreme Court clerks and lawyers showed that 57 percent thought Obamacare would be adjudged anti-constitutional.

But, as justice Ruth Bader Ginsburg noted recently: "At the Supreme Court, those who know, don't talk. And those who talk don't know."

For a storied court that has ruled to expand civil liberties, to end segregation in public schools, to give limited abortion rights to women, this week will see another landmark decision -- no matter which way it goes.