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 plan for mobilizing the federal government to achieve universal health care.
Washington state Attorney General Rob McKenna sounded confident Wednesday that the high court will discard the mandate to buy insurance he considers unconstitutional but keep much of the rest of the law.
"Based on the questions and comments justices made, supporters of the individual mandate are justifiably nervous," McKenna, a Republican, said a few hours after the justices concluded its third and final day of oral arguments in the case brought by 26 states, including Washington.
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 Ronald 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 Justice John Roberts and Justice Anthony 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 health care 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.
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.
McKenna made Washington part of the multi-state lawsuit primarily because of his opposition to the law's requirement that, starting in 2014, nearly every American buy health insurance or pay a fine. He's also said he doesn't think the entire law should be tossed out, though that is what the multi-state action seeks to do.
After reading some of the transcripts, McKenna said he "didn't hear much to suggest the majority of the court would want to throw out the whole law."
McClatchy News Services and Herald writer Jerry Cornfield contributed to this report.
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