WASHINGTON -- Here's a thought that can't comfort President Barack Obama: The fate of his health care overhaul rests with four Republican-appointed Supreme Court justices.

His most sweeping domestic achievement could be struck down if they stand together with Justice Clarence Thomas, another GOP appointee who is the likeliest to vote against.

The good news for Obama is that he probably needs only one of the four to side with him to win approval of the law's crucial centerpiece: the requirement that almost everyone has insurance or pays a penalty.

Lawyers with opposing views on the issue uniformly agree that the four Democrat-appointed justices, including Obama's two picks, Elena Kagan and Sonia Sotomayor, will have no trouble concluding that Congress did not overstep its authority in adopting the insurance requirement that is aimed at sharply reducing the now 50 million people without insurance.

On the other side, Thomas has made clear in several cases that he does not take an expansive view of Congress' powers.

Both the Obama administration and the health care law's challengers believe they can attract the other four Republicans to their side. The group includes Chief Justice John Roberts and Justice Samuel Alito -- the two appointees of President George W. Bush who have swung the court to the right in a number of areas -- conservative stalwart Justice Antonin Scalia and the less doctrinaire Justice Anthony Kennedy.

There is no consensus about which way the court will go or even how each of those four justices will vote. The court has set aside six hours over three days, beginning March 26, to hear arguments.

A legal challenge, once seen as improbable at best, now has everyone's attention, partly because the justices find it weighty enough to devote six hours over three days to hearing the case. That's the most time for any issue in more than 45 years.

"Arguments that once seemed outlandish don't seem quite so outlandish anymore," said University of Michigan law professor Nicholas Bagley, a health law expert who says the law should be upheld.

The fight over the law has played out in starkly partisan terms. It passed Congress without a Republican vote. All the GOP presidential candidates have called for its repeal.

Some supporters of the law worry about the high court's decision because a similar partisan split, with a few important exceptions, has emerged in the lower courts.

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