The US Supreme Court is set to deliver its ruling on President Barack Obama's 2010 health care overhaul, his signature domestic policy achievement.
The historic judgment on Thursday could hand him a huge triumph or a stinging rebuke just over four months before he seeks re-election.
The nine justices are scheduled to take the bench at 10am (1400 GMT) on the last day of the high court's term to read their final opinions, including their decision in the epic legal battle over the health care law.
The US health care system's biggest overhaul in nearly 50 years, the law aimed to provide medical insurance to more than 30 million previously uninsured Americans and to slow down soaring medical costs.
The US spends more on health care than any other nation, but about 50 million of the roughly 310 million Americans have no insurance at all.
Obama seeks re-election on November 6 against Republican challenger Mitt Romney, who has called for scrapping the law and replacing it with other measures even though he championed a similar approach at the state level as Massachusetts governor.
Republicans and Democrats alike have eagerly awaited the ruling and are sure to try to use the court's decision - whatever it may be - to their political advantage in the coming months.
A key component of the law was challenged by 26 of the 50 states and by a trade group for small businesses on the grounds that Congress exceeded its powers under the US Constitution by requiring people to obtain insurance by 2014 or pay a penalty.
The justices could uphold the entire law, strike down certain provisions or overturn the whole law in a ruling that could reshape the federal government's powers versus the states.
"If the court invalidates any of the law's provisions, it could be one of the most important federalism rulings from the court - and one of the most dramatic confrontations between the court and the president - since the 1930s," said Daniel Conkle, an Indiana University law school professor.
Obama and his fellow Democrats expended a great deal of energy and political capital in securing congressional passage of the measure over unified Republican opposition.
The law is reviled by conservatives, who dubbed it "Obamacare". Critics say it meddles in people's lives and in the business of the states.
The ruling also will be closely watched on Wall Street, with wide ramifications for the health sector. It could affect the stock prices of health insurers, drugmakers, device companies and hospitals.
The decision could be a defining moment for Obama's presidency and for Chief Justice John Roberts, who was appointed to the Supreme Court in 2005 by then-President George Bush.
In the past, Roberts often has endorsed more narrow rulings that bridge the ideological differences on the closely divided court, on which nine justices are split ideologically and politically.
Roberts and the four other conservatives, who often make up a majority, were appointed by Republican presidents.
The four liberals, who often dissent, were named by Democratic presidents, including Obama.
The health care battle has been the most politically charged case before the Supreme Court since 2000, when the justices halted the Florida vote recount in a ruling that gave the Republican Bush the presidency over Democrat Al Gore.
About 56 per cent of Americans said they opposed the healthcare law in a Reuters/Ipsos poll released on Sunday.
When asked about individual provisions, however, most respondents said they strongly supported them, except for the individual insurance purchase mandate, which was opposed by 61 per cent of those surveyed.
On other parts of the law, 82 per cent favored banning insurers from denying coverage to people with pre-existing medical conditions; 61 per cent backed letting children stay on their parents' insurance until age 26; and 72 per cent supported requiring companies with more than 50 workers to offer insurance.
The Supreme Court's mixed ruling on Monday on Arizona's crackdown on illegal immigrants could foreshadow a similar conciliatory result in health care that does not fall along predictable ideological lines.
The justices upheld police stops of people suspected of being in the country illegally, but struck down three other provisions of the state law in a ruling that left Obama and Arizona's Republican Governor Jan Brewer both able to claim partial victories.
The immigration and health care cases involved battles that pitted the Democratic Obama administration against Republican-led states and tested where to draw the line in the division of powers between the federal government and the states.