The nine justices are scheduled to take the bench at 10 a.m. (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 healthcare law.

The U.S. healthcare 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 United States spends more on healthcare 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 eagerly awaited the ruling and were 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 U.S. 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.

A DEFINING MOMENT

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 W. Bush. In the past, Roberts often has endorsed more narrow rulings that bridge the ideological differences on the closely divided court.

The 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 healthcare 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 percent 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 percent of those surveyed.

On other parts of the law, 82 percent favored banning insurers from denying coverage to people with pre-existing medical conditions; 61 percent backed letting children stay on their parents' insurance until age 26; and 72 percent 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 healthcare 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 able to claim partial victories.

The immigration and healthcare 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.

The Supreme Court cases are National Federation of Independent Business v. Sebelius, No 11-393; U.S. Department of Health and Human Services v. Florida, No. 11-398; and Florida v. Department of Health and Human Services, No. 11-400.

(Additional reporting by Joan Biskupic; Editing by Howard Goller and Will Dunham)

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