Even a high-risk insurance pool created under the law to help people with pre-existing conditions who were without insurance at least six months will expire in January 2014, when the broader changes were to take effect. About 60,000 people have joined that plan, the Health and Human Services Department says.
Under this scenario, states could seek to fill the gap with their own mandates and insurance market changes. That's most likely in states with Democratic governors and legislatures.
Democrats in Congress also could try to replace the mandate and insurance market overhaul with lesser changes.
Among opponents, however, "there's going to be blood in the water," Cannon says. Opponents "are going to be energized to get rid of the rest of it."
House Republicans say they will try to repeal the rest of the law — its insurance expansions, taxes, Medicare savings and more — and then vote on lesser fixes as part of their "repeal and replace" strategy.
"Our goal would be to have the end result be all of it gone," says Brendan Buck, a spokesman for House Speaker John Boehner. "Clear out whatever is left from what they strike, and then move step by step on our replacement provisions."
Option 4: Law killed
The justices may decide to strike down all the taxes, subsidies, mandates, coverage expansions, insurance market changes and other provisions in the law. But some may find their way back.
Several insurance companies plan to keep some changes already enacted. "Consumers and employers will continue to have the option of purchasing coverage that includes many of the benefits they have today, such as allowing dependent children to stay on their parents' policies until age 26," says Karen Ignagni, president of America's Health Insurance Plans.
Some 6.6 million young adults joined their parents' plans in the first year of eligibility, according to the Commonwealth Fund, a liberal health research group.
One program at risk would be discounts from drug companies for more than 5 million seniors and people with disabilities. The Medicare discounts have saved consumers $3.5 billion since the law was passed.
The bigger question: Would the effect of the court's ruling be prospective or retroactive? It would be difficult to retract benefits already on the books.
"You would have a mess on your hands," says Levitt, of the Kaiser Family Foundation. "The general sense is anything that's already happened kind of is water under the bridge."
The politics could be a wash, as in the case of the law being upheld. While Obama and Democrats would lose their most prized achievement from his term in office, their liberal base would be energized. A recent United Technologies/National Journal poll found striking down the law would have little impact on the president's re-election
The Supreme Court, favored by just 44% of Americans in a recent New York Times/CBS Poll, could become more unpopular after such a perceived power grab.
"You could well imagine an argument being made around the politicization of the court," says Sheila Burke, a health policy lecturer at Harvard University who served as the top aide to former Senate Republican leader Bob Dole.
What would happen next?
Democrats might seek to replace at least portions of the law, such as smaller federal subsidies for the uninsured. But they would face the "Obamacare" moniker that has made the law relatively unpopular, Cannon says.
Republicans likely would follow through on their "repeal and replace" agenda by proposing lesser changes. Romney, for instance, wants to give states more flexibility, let consumers buy insurance across state lines, and limit awards in medical malpractice lawsuits.
"It puts the ball back in the political arena," says Randy Barnett, a Georgetown University law professor who represents the business groups challenging the law. "We'll have a national debate about health care reform, and then we'll have an election, and whoever wins the election will have a mandate."