Making health plans available and affordable to people who buy their own policies was a key goal for backers of the federal law known as Obamacare.
But if the Supreme Court strikes down the insurance subsidies of millions of Americans who rely on the federal marketplace, it could leave many worse off than they were before the law took effect.
"The doomsday scenario could materialize, and it does impact everyone," said lawyer Christopher Condeluci, who worked for Iowa Republican Charles Grassley on the Senate Finance Committee staff during the law's drafting. Those affected would include individuals receiving subsidies and those paying the full cost of their plans in states such as Pennsylvania and New Jersey that use the federal exchange.
Millions of consumers would likely be unable to afford their policies without subsidies.
Most insurers could not drop plans without giving one to three months' notice. But the companies remaining in the market would likely seek sharp premium increases for the next year, anticipating that more of the sickest consumers would hold onto their plans.
An analysis by the RAND Corp. projects that 70 percent of consumers would cancel their policies. The premium increases, in turn, could drive more people to drop coverage, spurring further price increases and potentially leading to what experts call "a market death spiral."
"It's not the subsidy market that will fall apart, it's the whole market" for everyone who doesn't get job-based insurance coverage, said Robert Laszewski, an insurance industry consultant who is no fan of the health law. "There will be millions of Republicans who are not subsidy-eligible who are also going to get screwed."
At issue in King v. Burwell - slated to be argued before the Supreme Court on Wednesday - are the subsidies that go to millions of low- and moderate-income Americans in 37 states that rely on the federal healthcare.gov website. More than 85 percent of the 8.6 million people who bought plans in those states qualified for subsidies, officials say.
The law's challengers point to four words in the Affordable Care Act that say subsidies shall be distributed through marketplaces "established by the state." They argue that that wording bars the government from subsidizing insurance bought through a federal exchange. Supporters of the law argue that Congress intended the subsidies to be available through both federally run and state-run markets, which they say is clear in reading the overall bill.
The ruling would have no effect on the subsidies given to residents through the 13 state-run markets, such as the one in New York.
The Obama administration has declined to discuss contingency plans, expressing confidence that it will prevail with the justices. "Congress would not pass a law that 87 percent of folks would not get subsidies but people in say, New York, would," Health and Human Services Secretary Sylvia Mathews Burwell said Wednesday.
Experts say Congress could also apply "fixes," such as voting to allow subsidies to continue through the rest of the year. But whether a Republican-led Congress that has pledged itself to the law's repeal would agree to that is uncertain.
Aetna spokeswoman Cynthia Michener said the insurer was talking with lawmakers from both parties "about how to make a grand bargain should the Supreme Court decide against federal exchange subsidies." A decision to strike the subsidies would likely "spur bipartisan action to resolve the issue promptly," she added.
In Pennsylvania, the Wolf administration is working "to make sure that a state-based exchange is an option" no matter how the Supreme Court rules, said spokesman Jeff Sheridan.
Joel Ario, a managing director at the consultancy Manatt Health Solutions, estimated that one-third of states would set up their own markets fairly quickly. If states move at the same rate as they have to expand Medicaid, it could take several years before most states have their own markets, said Ario, a former Pennsylvania insurance commissioner.
Many insurers do not yet have contingency plans, he said, partly because the future is so unclear. "There really isn't a contingency plan for nuclear destruction," Laszewski said.
Others don't see a ruling in such dark terms.
"The Supreme Court generally doesn't go out of its way to wreck the economy or the health system," said Stuart Butler, a conservative scholar at the Brookings Institution.
Kaiser Health News is an editorially independent program of the Kaiser Family Foundation.