WASHINGTON (AP) — The U.S. Supreme Court on Monday promised an extraordinarily thorough springtime review of President Barack Obama's historic health care overhaul — more than five hours of argument, unprecedented in modern times — in time for a likely ruling affecting millions of Americans just before the presidential election in November.

That ruling, expected before next summer's July 4 holiday, could determine the fate of Obama's signature domestic achievement, the most far reaching domestic legislation in a generation but a political lightning rod as well. It is vigorously opposed by all of Obama's prospective Republican opponents.

The Patient Protection and Affordable Care Act aims to provide health insurance to more than 30 million previously uninsured Americans. But Republicans have branded the law unconstitutional since before Obama signed it in a ceremony in March 2010.

The court's ruling could be its most significant and political decision since George W. Bush's 2000 presidential election victory. But the justices left themselves an opening to defer the outcome if they choose, by requesting arguments on one lower court's ruling that a decision must wait until 2015, when one of the law's many provisions takes effect.

Legal experts have offered a range of opinions about what the high court might do. Many prominent Supreme Court lawyers believe the law will be upheld by a lopsided vote, with Republican and Democratic appointees ruling in its favor. But others predict a close outcome, with Justice Anthony Kennedy, a Republican who sometimes joins his four Democratic colleagues, holding the deciding vote.

The White House has pushed for a final ruling as soon as possible, and Communications Director Dan Pfeiffer said the administration was pleased the justices agreed to take the case now, with arguments in March. "It's important that we put to rest once and for all the issue of maybe the law will disappear," said Health and Human Services Secretary Kathleen Sebelius.

Republicans also said they were happy the high court would hear arguments on the constitutionality of the provision at the heart of the law and three other questions about the act. The central provision in question is the requirement that individuals buy health insurance starting in 2014 or pay a penalty.

"That the Supreme Court is taking this up, I think, is a positive signal that there are legitimate concerns surrounding the constitutional aspects of mandating that individuals purchase health care insurance and purchase it according to Washington's guidelines," said House Majority Leader Eric Cantor, a Republican.

The exceptional five and a half hours allotted for argument demonstrates the significance the justices see in this case. Normally, they allow only one hour, split between two sides. In the modern era, the last time the court increased that time anywhere near this much was in 2003 for consideration of the McCain-Feingold campaign finance overhaul. That case consumed four hours of argument.

This argument may even spread over two days, as the justices rarely hear more than two or three hours a day.

The health care overhaul would achieve its huge expansion of coverage by requiring individuals to buy health insurance starting in 2014, by expanding medical aid for the poor Medicaid and by applying other provisions, many yet to take effect.

The central question before the court is whether the government has the power to force Americans to buy health insurance. The White House says Congress used a "quintessential" power — its constitutional ability to regulate interstate commerce, including the health care industry — when it passed the overhaul.

But opponents of the law, and the 11th U.S. Circuit Court of Appeals in Atlanta, say that Congress overstepped its authority when lawmakers passed individual mandate. A divided Atlanta court panel ruled that Congress cannot require people to "enter into contracts with private insurance companies for the purchase of an expensive product from the time they are born until the time they die."

The Atlanta court is the only one of four appellate courts that found the mandate unconstitutional. The federal appeals court in Cincinnati upheld the entire law, as did appellate judges in Washington in recent days. The appellate court in Richmond, Virginia, ruled a judicial decision on the law cannot be rendered until 2015, after the penalties for not having insurance have gone into effect.

Supporters have been encouraged that the appellate rulings in Cincinnati and Washington to uphold the law were joined by two prominent conservative judges appointed by Republican presidents: Jeffrey Sutton, appointed by George W. Bush, and Laurence Silberman, appointed by Ronald Reagan.

In Atlanta, however, Frank Hull, appointed by President Bill Clinton, joined with a Republican colleague in striking down the mandate.

Earlier District Court rulings followed political affiliation: Judges appointed by Democratic presidents upheld the law, while Republican appointees struck it down.

In addition to deciding the constitutionality of the central mandate, the justices also will determine whether the rest of the law can take effect even if that core is held unconstitutional. The law's opponents say the whole thing should fall if the individual mandate falls.

The administration counters that most of the law still could function, but says that requirements that insurers cover anyone and not set higher rates for people with pre-existing conditions are inextricably linked with the mandate and shouldn't remain in place without it.