But as the court considered whether the federal government could require most Americans to get health insurance, Kennedy appeared to deal the president and his allies a heavy blow.
The mandate, he said, "changes the relationship of the federal government to the individual in a very fundamental way." Kennedy called the insurance requirement "concerning" and suggested it might be "unprecedented."
Predicting decisions based on justices' comments during arguments can be risky. And on the second day of oral arguments over the landmark law, Kennedy and Chief JusticeJohn G. Roberts Jr.at times seemed to agree with the government's view that because everyone is likely to need medical care at some time, Congress might have more latitude to require purchase of insurance than of some other product.
But those moments were exceptions. Overall, the clear skepticism about the insurance mandate expressed by Roberts and Kennedy seemed to set the stage for a 5-4 decision, with the court's five Republican appointees pitted against its four Democrats.
Such a ruling would strike out the heart of Obama's healthcare law, his signature domestic achievement, in the middle of the presidential campaign, which could become a significant, and unpredictable, factor in the election. Not since the mid-1930s has the Supreme Court struck down a major regulatory act of Congress.
A ruling against the mandate would not necessarily overturn all of the healthcare law. The question of how much might be left is one the justices will grapple with in Wednesday's arguments, the finale of three days of debate over the law. But on Tuesday, at least, the administration's supporters were dispirited.
"It certainly didn't go as well as I had hoped it would for the government," said Timothy Jost, a law professor at Washington and Lee University and an authority on healthcare law. "It was always fairly certain they had four votes. It's clear now they are going to have a hard time pulling a fifth."
Kennedy and Roberts, along with fellow conservative Justices Antonin Scalia and Samuel A. Alito Jr., repeatedly questioned where the limit on federal power would be if the mandate was upheld. Justice Clarence Thomas was silent, as is his habit, but is expected to vote to strike down the mandate.
"If the government can do this, what else can it … do?" Scalia asked, suggesting Congress might require Americans to buy broccoli or automobiles.
Roberts suggested that the government might require Americans to buy cellphones to be ready for emergencies.
Kennedy, who is often the court's swing vote, seemed to suggest that a mandate directed at individuals could be upheld only if the government offered an extremely powerful justification. And his comments from the bench raised considerable doubt about whether he thought the administration had met that test.
Solicitor Gen. Donald Verrilli Jr., the Obama administration's top lawyer, tried to argue that the insurance mandate would not open the door to other requirements to buy products because healthcare is unique.
"Virtually everybody in society is in this market," said Verrilli, who appeared to struggle to answer conservative justices' probing questions. Justice Ruth Bader Ginsburg and other liberal justices intervened at points to help him along, defending the new law as a reasonable means to cope with the uninsured.
From the start, the mandate to buy insurance has been seen as the central and most controversial provision in the Patient Protection and Affordable Care Act. It is by far the one most Americans say they oppose, according to polls.
Under the law, many of the nearly 20% of individuals in the U.S. who do not have health insurance will have to sign up for a plan starting in 2014 that meets a basic set of standards or pay a tax penalty that will rise from $95 in 2014 to $695 in 2016. Health policy experts warn that without some incentive to buy insurance, people could wait until they got seriously ill to sign up for coverage, pushing up premiums for everyone.
In defense of the mandate, Verrilli said that if a person elected not to get health insurance but then got sick — as nearly everyone will at some point — that person would pass along costs to everyone else.
To prevent that, the Obama administration has argued that Congress can use its authority under the commerce clause of the Constitution to impose the mandate as a way to regulate health insurance. Roberts and Kennedy in the past have supported the federal government's broad authority to regulate commerce.
The Constitution says Congress has the power to "regulate commerce" and to impose taxes to promote the general welfare. The court has upheld federal laws regulating all manner of business, including agriculture, aviation and who can be served at the corner coffee shop.
The few positive moments for the administration came when the law's opponents got their turn to argue. Kennedy and Roberts pressed them to address the unique dynamics of the healthcare market, in which people who do not get health coverage affect everyone's costs.
"That is not true in other industries," Kennedy said at one point. "That's my concern in the case."
In his closing comments, Verrilli shifted gears and tried to convince the conservative justices of the virtues of judicial restraint.
"The Constitution leaves the democratically accountable branches of government" the choice of how best to regulate business and commerce, he said. "That is exactly the kind of thing that ought to be left to the judgment of Congress."
After Wednesday's third day of arguments, the justices will meet behind closed doors Friday to cast their votes and begin working on opinions.
