The court will now take up whether any remnant of the reform can survive if that linchpin fails.
The justices' questions in Tuesday's hearing carried serious implications but were sometimes flavored with fanciful suggestions. If the government can force people to buy health insurance, justices wanted to know, can it require people to buy cell phones? Broccoli?
The law would affect nearly all Americans and extend insurance coverage to 30 million people who now lack it. The court focused on whether the mandate for Americans to have insurance "is a step beyond what our cases allow," in the words of Justice Anthony Kennedy.
But Kennedy, who is often the swing vote on cases that divide the justices along ideological lines, also said he recognized the magnitude of the nation's health care problems and seemed to suggest they would require a comprehensive solution.
He and Chief Justice John Roberts emerged as the apparent pivotal votes in the court's decision. The ruling is due in June in the midst of a presidential election campaign that has focused in part on the new law.
Today's final arguments -- the third day of hearings -- are to focus on whether the rest of the law can remain even if the insurance mandate is struck down and, separately, on the constitutionality of another provision expanding the federal-state Medicaid program.
The insurance requirement is intended to complement two unchallenged provisions of the law that require insurers to cover people regardless of existing medical conditions and limit how much they can charge in premiums based on a person's age or health.
The law envisions that insurers will be able to accommodate older and sicker people without facing financial ruin because the insurance requirement will provide insurance companies with more premiums from healthy people to cover the increased costs of care.
'A heavy burden'The biggest issue, to which the justices returned repeatedly during two hours of arguments, was whether the government can force people to buy insurance.
Justices Antonin Scalia and Samuel Alito appear likely to join with Justice Clarence Thomas -- the only justice who didn't ask any questions -- to vote to strike down the key provision of the overhaul. The four Democratic appointees seemed ready to vote to uphold it.
Roberts also spoke about the uniqueness of health care, which almost everyone will use.
"Everybody is in this market, so that makes it very different than the market for cars or the other hypotheticals that you came up with, and all they're regulating is how you pay for it," Roberts said, paraphrasing the government's argument.
Everyone is affectedSolicitor General Donald Verrilli Jr. sought to assure the court that the insurance mandate in the Patient Protection and Affordable Care Act that Obama signed into law in 2010 is a key part of the law's goal of reaching many of the more than 40 million people who don't have health insurance through their employers, don't qualify for government aid and cannot afford to buy coverage on their own.
Paul Clement, who is representing Florida and 25 other states -- including Michigan -- in challenging the law, called the mandate "an unprecedented effort by Congress."
Clement said the requirement would force people, especially those who are young and healthy, to buy a product they don't want.
Michael Carvin, representing the National Federation of Independent Business in opposing the law, also pushed hard on the notion of individual freedom. When Justice Stephen Breyer asked whether the federal government could order vaccinations "if there was some terrible epidemic sweeping the United States," Carvin said Congress lacks the power to do so.
Justice Ruth Bader Ginsburg said she found the debate over health care similar to an earlier era's argument about the Social Security retirement system. How could Congress be able to compel younger workers to contribute to Social Security but be limited in its ability to address health care? she wondered.
"There's something very odd about that, that the government can take over the whole thing and we all say, 'Oh, yes, that's fine,' but if the government wants to preserve private insurers, it can't do that," she said.
Scalia and Roberts noted that the health care overhaul law would make people get insurance for things they may not need, such as heart transplants or pregnancy services. "You can't say that everybody is going to participate in substance abuse services," Roberts said.
On the other hand, Ginsburg said, "The people who don't participate in this market are making it more expensive for those who do."