Conservative justices of the US Supreme Court have questioned whether the US government has the power to penalise Americans who have no medical cover.
The weighty question of the provision at the core of President Barack Obama's 2010 healthcare reform was under the microscope on day two of the hearing.
The nine judges spent about two hours grilling attorneys on the hotly disputed individual mandate.
A ruling on the politically explosive issue is expected by late June.
That decision would come right in the thick of the campaign for November's presidential and congressional elections.
The legal challenge has been brought by 26 US states which say the individual mandate violates the principles of freedom and liberty enshrined in the US constitution.
Backers of the law see the provision, which does not take effect until 2014, as crucial for reducing the numbers of Americans living without health insurance.
As the latest session got under way, protesters for and against the law once again held demonstrations on the steps of the court in Washington DC, reflecting the bitterly divisive passions aroused by the law.
Congresswoman Michele Bachmann, a former Republican presidential candidate, was joining a Tea Party rally held nearby to decry the law.
The four liberal justices, Stephen Breyer, Ruth Bader Ginsburg, Sonia Sotomayor and Elena Kagan, all indicated on Tuesday that they believed the mandate was valid under the US constitution.
But Republican-appointed judges, Chief Justice John Roberts, along with Justices Antonin Scalia, Samuel Alito and Anthony Kennedy, fired a barrage of sceptical questions at Solicitor General Donald Verrilli.
He was asked whether, if citizens were made to purchase medical cover, could the federal government require them to buy certain cars, gym membership or broccoli.
The solicitor general argued that the reforms fell within Congress' rights under the US constitution to regulate interstate commerce.
But Justice Kennedy asked: "Can you create commerce in order to regulate it?"
Justice Ginsburg observed that "the people who don't participate in the market are making it more expensive for those that do".
Justice Scalia fired back: "You could say that about buying a car. If people don't buy a car, the price [car-buyers] will pay will be more."
The Obama administration argues those without such health insurance foist their unpaid medical bills on to taxpayers, who are forced to subsidise emergency room visits.
The Patient Protection and Affordable Care Act, passed in March 2010, has been the controversial centrepiece of President Obama's term in office.
The administration estimates it will bring 32 million Americans into the healthcare system and reduce the cost on those who already buy insurance.
If upheld, the law would forbid insurance companies from denying coverage because of pre-existing medical conditions. It would also limit how much they can charge older people.
On Monday, the justices appeared to satisfy themselves they had the jurisdiction to review the law even though it has not yet taken effect.
On Wednesday, two questions will be considered.
The first is whether, if the individual mandate is declared unconstitutional, the rest of the law can stand or must 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.
The US was the only major developed country without a national healthcare system until President Obama's reform.
An opinion poll published on Monday found that 47% of voters disapproved of the healthcare law and 36% were in favour. Republicans seeking to foil President Obama's bid for a second term have all vowed to repeal it if elected.