For Republicans, a court decision invalidating part or all of Obamacare would be a powerful talking point, cited as a stinging rejection of the president's agenda and proof that Obama -- a former constitutional law professor -- doesn't understand or respect the Constitution, a cherished belief of the Tea Party. In policy terms, too, the president's central domestic accomplishment would suddenly cease to exist or else limp forward in badly maimed form.

"If the whole thing goes down, I think it's pretty tough for the president," said Howard Dean, the physician and former Democratic National Committee chairman. "I think he's probably going to win reelection anyway, but does he want to start all over again?" On the heels of a court rejection, he said, "I don't think [health-care reform] could come back. Not the whole thing." (Dean, however, believes the mandate isn't necessary and could be removed from the law without consequence.)

Opponents of the law agree: This is universal health care's Waterloo.

"This was their one big shot. They certainly thought so. They pulled out all the stops," said Douglas Holtz-Eakin, a conservative economist and president of the American Action Forum, which has filed court briefs in opposition to the health-care law.

Already, he said, the law was on shaky ground. "The vast public rejection of the law, its broad unpopularity, the fact that so many Republicans were elected on it in 2010, the fact that so many states are rejecting it -- really, it wasn't looking strong," he said.

Through all that, liberals took solace in the fact that at least the legislation was on the books, moving inexorably toward implementation; eventually, they were convinced, Americans would grow used to the new system, come to depend on it, and become unable to imagine life without it.

"But it's not done yet," Holtz-Eakin said. "And if they overturn it, it will not be done. In my lifetime, it doesn't get done. Nothing like this."

Liberals were shocked by the justices' willingness to call into question the legal basis for not only the insurance mandate included in the reform law, but less controversial concepts such as the federal government's ability to dictate the terms of Medicaid grants to the states.

"I was baffled the court would even hear that argument, let alone take it seriously," said Ron Pollack, executive director of the pro-reform Families USA and a health-care activist for three decades. If that goes, federal programs to improve education, the environment, transportation. and more could suddenly be under fire, he said -- creating the potential for the wholesale gutting of the New Deal and the welfare state.

In political terms, a rejection of all or part of the Affordable Care Act would dramatically reshape the playing field for the 2012 general election. With the law in place, the health-care debate is a referendum on an existing (if not fully implemented) piece of legislation that remains marginally unpopular and poorly understood. Mitt Romney, Obama's likely Republican opponent, has vowed to repeal the law, although his argument against it is muddied by his past support of insurance mandates. Nonetheless, by keeping the focus on Obamacare's perceived shortcomings, he would be making primarily a negative argument.