The US Supreme Court is to begin three days of landmark hearings on the constitutionality of President Barack Obama’s signature healthcare reform.
Twenty-six US states have brought the Republican-led challenge to the administration’s 2010 law.
The act expands health coverage to 32 million Americans, but critics say its requirement that people buy insurance intrudes on civil liberties.
A ruling is expected by late June – in the thick of the election season.
People have been camping outside the court in Washington DC since Friday.
The nine Supreme Court justices, five of them appointed by Republican presidents and four by Democrats, have allotted six hours of argument time – the longest in decades – for the case.
Because of the huge interest, the court will release daily audio recordings of the proceedings.
The broccoli question
It is one of the most politically explosive cases since the 2000 election wrangle that saw the White House awarded to Republican George W Bush over Democrat Al Gore, or the hearings on the 1974 Watergate tapes that led to President Richard Nixon’s resignation.
This Patient Protection and Affordable Care Act, passed in March 2010 to much fanfare by the Obama administration, is vigorously opposed by many conservatives.
If upheld, the law would forbid insurance companies from denying coverage due to pre-existing medical conditions. It would also limit how much they can charge older people.
But the most controversial aspect of the law is its core requirement that most people buy health insurance or pay a tax penalty.
Monday’s hearing will focus on whether the judges actually have the power to rule on the case.
They will hear 90 minutes of argument about an obscure 19th-century piece of legislation – the 1867 Anti-Injunction Act – which deters legal challenges to any tax law until it has taken effect.
The healthcare law’s insurance mandate does not begin until 2014, and those who remain uninsured would not have to pay a penalty until the following year.
The justices could decide they are unable to tackle the case, sparing them the task of ruling on such a hugely partisan issue with November’s general election looming.
The most eagerly anticipated day is Tuesday, when the judges hear arguments on the constitutionality of the individual insurance requirement.
The 26 states led by Florida say the federal government has no power to force individuals to buy a product such as insurance.
Critics warn that if Congress can require citizens to buy health insurance, they could also force consumers to eat broccoli, and purchase gym membership or buy American-made cars.
However, the Obama administration says that Americans who have no healthcare simply push their unpaid health bills upon taxpayers, who are forced to subsidise emergency room visits.
On Wednesday, two questions will be heard.
The first is whether, if the individual mandate is declared unconstitutional, the rest of the law can stand or must it be struck down in its entirety.
The other is whether Congress unfairly burdened states when it expanded eligibility under Medicaid, the medical care programme for poor people.