WASHINGTON — The Supreme Court, after three days of arguments on President Obama’s healthcare overhaul, appeared ready to strike down not just the requirement that individuals have insurance, but the entire law, invalidating a major piece of domestic legislation for the first time since the Depression.

That prospect, unthinkable to many experts as recently as last week, will not be certain until the justices rule on the case, probably in June.

But the electric set of arguments that ended Wednesday revealed profound skepticism about the law by the court’s five-member conservative majority, which appeared openly hostile to its scheme for mobilizing the federal government to achieve universal healthcare.

In question after question the nine justices revealed themselves as sharply divided as Congress and the American public over the virtues of Obama’s law.

But unlike Congress — whose slim Democratic majority allowed passage of the law in 2010 — the court has a narrow majority of Republican appointees. If united, the five conservatives could vote to strike down the entire law. And during arguments Wednesday, they sounded prepared to do just that, including scrapping a major expansion of the Medicaid health insurance program for the poor.

“One way or another, Congress is going to have to reconsider this,” said Justice Antonin Scalia, an appointee of President Reagan. “Why isn’t it better to have them reconsider it in toto?”

Justice Ruth Bader Ginsburg, the senior liberal, appealed for a more cautious approach. She said the court should do a “salvage job” rather than undertake a “wrecking operation.” But she looked to be outvoted by the conservatives.

The justices will meet in private Friday to cast their votes. Their comments during oral arguments do not always forecast the eventual ruling. And it has been widely assumed that the justices, particularly Chief JusticeJohn G. Roberts Jr.and JusticeAnthony M. Kennedy, would try mightily to avoid striking down a major law by a 5-4 vote. Solicitor Gen. Donald Verrilli Jr., the Obama administration’s top lawyer, pleaded with the court for restraint Wednesday.

“Congress struggled with the issue of how to deal with this profound problem of 40 million people without healthcare for many years. And it made a judgment,” Verrilli said as he wrapped up his arguments.

“This was a judgment of policy that democratically accountable branches of this government made by their best lights, and I would encourage this court to respect that judgment,” he said.

The cause was taken up by the court’s four liberal members, who urged their colleagues not to throw out the entire law, even if they find the provision requiring Americans to have health insurance unconstitutional.

“What’s wrong with leaving it … in the hands of the people who should be fixing this, not us?” asked Justice Sonia Sotomayor.

The administration was prepared to accept a ruling that if the mandate was struck down, some other provisions should fall as well, such as the requirement that insurers sell coverage to people with preexisting conditions.

But there were few signs, if any, that any of the five conservatives were persuaded, including Kennedy, long considered the court’s swing vote.

Like the other conservatives, Kennedy signaled deep unease with the provision that would require nearly all Americans to have health insurance starting in 2014.

On Wednesday, he seemed reluctant to excise that provision alone from the sweeping law for fear of harming the economic interests of the insurance companies, which he said would be “a more extreme exercise of judicial power … than striking the whole.”

Scalia appeared openly scornful of the suggestion that Congress could fix the law if the court threw out the insurance mandate. “Don’t you think it’s unrealistic to say leave it to Congress, as though you’re sending it back to Congress for Congress to consider it dispassionately?” he said.

Roberts and Justice Samuel A. AlitoJr.appeared equally leery. Justice Clarence Thomas, the fifth member of the conservative wing, did not ask any questions but is widely seen as eager to invalidate the Patient Protection and Affordable Care Act.

On Wednesday, the conservative justices targeted a provision that provides funding for states to enroll 17 million more people in their Medicaid programs over the next decade, a key pillar of the law’s program for expanding coverage.

Kennedy seemed to accept the argument by 26 states challenging the law that they are being unjustly forced to administer a massive Medicaid expansion.

Verrilli tried to persuade the justices that the law’s provision providing hundreds of billions of dollars to states to expand their Medicaid programs in 2014 was not “coercive.”

Under the act, the federal government would pay more than 90% of the cost of expanding Medicaid to cover all poor Americans, while each state would retain responsibility for administering its own program, as has been the case since Medicaid was created in 1965.

That is a “big gift,” said Justice Elena Kagan, who was joined by the three other liberal members of the court in defending the Medicaid expansion.

But the conservative justices appeared far more in line with Paul Clement, the attorney for the states. He argued that the aid would undermine state authority by leaving states effectively no choice but to expand Medicaid or face the possibility that the secretary of Health and Human Services would cut off all their aid.

“Why shouldn’t we be concerned about the extent of authority that the government is exercising, simply because they could do something less?” Roberts said, brushing aside Verrilli’s efforts to assure the court that no administration would actually take such a step.

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