The overhaul would lurch ahead, experts say.
But it would make an already complicated law a lot harder to carry out, risking repercussions for a U.S. health care system widely seen as wasteful, unaffordable and unable to deliver consistently high quality.
Premiums could jump for people buying coverage individually, and for small businesses. That's because other provisions of the law require insurance companies to accept people with health problems, and limit the premiums that can be charged to older adults.
Sooner or later, the dilemma of the nation's 50 million uninsured would land back on the doorstep of Congress.
During Tuesday's oral arguments, the Supreme Court's conservative justices fired off sharp, skeptical questions about the constitutionality of the mandate, fanning speculation that it may not withstand review.
Such an insurance requirement is unprecedented in federal law, but it is not the only lever for expanding coverage in Obama's legislation.
"The hyperbolic language that is being used about this is way over the top," said economist Gail Wilensky, who ran Medicare and Medicaid under President George H.W. Bush. The mandate "is important, but not that important. There are other strategies to encourage people to purchase health insurance."
Even if the mandate is struck down, the law's Medicaid expansion would still be carried out under a separate provision.
Starting in 2014, Medicaid would provide health insurance to over half the estimated 30 million people receiving coverage as a result of the law, mainly childless adults living near poverty.
Another provision in the law provides government subsidies for many middle-class people to purchase individual policies in new state markets. Also available starting in 2014, those subsidies would probably entice many to buy a plan.
And yet another part of the law imposes fines on medium-sized and large employers who do not provide coverage to their workers.
Still, various economic studies have projected that without the mandate ten million to 15 million people who would have been covered instead will remain uninsured.
Wilensky said the government would have options. It could impose penalties on people who postpone getting health insurance until they have a medical problem, higher premiums for instance.
"You don't have to buy health insurance, but we're going to make you pay for the cost you're imposing on the rest of us," said Wilensky. That's the approach Medicare uses—successfully—to get seniors to buy outpatient and prescription coverage.
If the Supreme Court strikes down the mandate, the Obama administration and the insurance industry have asked the justices to also invalidate consumer protections such as the law's ban on denying coverage to people with pre-existing health problems. Unless everybody is required to be in the pool, they argue, those safeguards won't work as intended, and could destabilize the insurance market.
Experts debate whether or not such a dire consequence will come about—or if coverage will just get even more expensive than it already is.
"Without a mandate the law is a lot less effective," says MIT economist Jonathan Gruber, who advised the Obama administration and, earlier on, then-Massachusetts Gov. Mitt Romney, who put such an insurance mandate in that state's health care law. "The market will not collapse, but it will be a ton more expensive and cover many fewer people."