Washington ? The 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.

That ruling, expected before next summer’s Independence Day 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 GOP 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 appointee who sometimes joins his four colleagues appointed by Democratic presidents, holding the deciding vote on the nine-member court.

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.

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.