Justices to hear arguments over bill that was signed into law in March 2010 – setting up election-year showdown with president

The Supreme Court has said it will hear arguments next March over President Barack Obama's healthcare overhaul, in a case that could shake the political landscape just as American voters are deciding if Obama deserves another term.

The decision to hear arguments in the spring allows plenty of time for a decision in late June, just over four months before election day. This sets up an election-year showdown over the White House's main domestic policy achievement that aims to extend medical insurance coverage to more than 30 million Americans.

The case could become the high court's most significant and political ruling since its 5-4 decision in the Bush v Gore case nearly 11 years ago effectively sealed George Bush's 2000 presidential election victory.

The justices announced they will hear more than five hours of arguments from lawyers – an extraordinarily long session – on the constitutionality of a provision at the heart of the 2010 law, and other related questions about the act. The central provision in question is the requirement that, starting in 2014, Americans buy health insurance – or pay a penalty.

The last time the court allotted anywhere near this much time for arguments was in 2003, for the consideration of the McCain-Feingold campaign finance reform. That case consumed four hours of argument.

A White House spokesman said, "We are pleased that the court has agreed to hear this case."

"We know the Affordable Care Act is constitutional, and are confident the Supreme Court will agree," communications direct Dan Pfeiffer said in a statement.

Senate Republican leader Mitch McConnell called the law an "unprecedented and unconstitutional expansion of the federal government into the daily lives of every American."

"In both public surveys and at the ballot box, Americans have rejected the law's mandate that they must buy government-approved health insurance, and I hope the supreme court will do the same," McConnell said.

Republicans have called the Patient Protection and Affordable Care Act unconstitutional since before Obama signed it into law in March 2010. But only one of the four federal appeals courts that have considered the health care overhaul has struck down even a part of the law.

The federal appeals court in Atlanta said Congress exceeded its power under the US constitution when it adopted the mandate. The federal appeals court in Cincinnati upheld the entire law, as did appellate judges in Washington in recent days.

In addition to deciding whether the law's central mandate is constitutional, the justices will also determine whether the rest of the law can take effect even if that central mandate 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 medical conditions are inextricably linked with the mandate and should not remain in place without it.

"The court recognised the seriousness of these vitally important constitutional challenges by allocating an extraordinary amount of time for oral argument," Florida attorney general Pam Bondi said.

Lastly, the justices might not settle the fight over the constitutionality of the law at all if they agree with a decision by the federal appeals court in Richmond, Virginia. Those appellate judges said arguments over the law's validity are premature because a federal law generally prohibits challenges to taxes until the taxes are paid. The appeals court reasoned that the penalty for not purchasing insurance will not be paid before federal income tax returns are due in April 2015.

The administration agreed to seek prompt supreme court review of the health care overhaul, though it had options for trying to delay the court's consideration of the law until after the election. The Justice Department passed up the chance to ask the appeals court in Atlanta to reconsider its decision. It is common for the Justice Department to seek review by the full appeals court when a three-judge panel rules against the government.

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

Compounding the speculation over the outcome is the backdrop against which a decision is expected, the final few months of the presidential campaign.