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 Independence Day, 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.

The overhaul 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 justices left themselves an opening to defer the outcome if they choose, by requesting arguments on a lower court’s ruling that a decision must wait until 2015, when one of the law’s many provisions takes effect.

Legal experts offered a range of opinions about what the court might do. Many prominent Supreme Court attorneys believe the law will be upheld by a lopsided vote, with Republican and Democratic appointees ruling in its favor. Others predict a close vote, with Justice Anthony Kennedy — a GOP 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, 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 said they are happy the 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, R-Va.

The 51/2 hours allotted for argument demonstrates the significance justices see in this case. Normally, they allow one hour, split between two sides. In the modern era, the last time the court increased that time near this much — four hours — was in 2003 for consideration of the McCain-Feingold campaign finance overhaul.