Cigna Corp., the fourth-largest U.S. insurer, said in an e- mailed statement it would wait until after the court rules to announce its plans. The Bloomfield, Connecticut-based company said it's "prepared to proceed as appropriate on behalf of our customers when the court deliberations reach their conclusion."

The ruling will mark the first time the high court has ruled on a president's signature legislative achievement in the middle of his re-election bid.

Obama's almost $1 trillion, 10-year plan to overhaul the health system passed Congress without a single Republican vote. Parts of the law expanding insurance coverage were challenged as unconstitutional by 26 states, and the Republican-led House of Representatives has voted at least 29 times to repeal all or part of the law or reduce its funding.

Republican Lawmaker

The head of a caucus of 21 Republican lawmakers with medical backgrounds said last week that no matter the outcome of the court case, he will try to preserve coverage for young adults and for people with pre-existing medical conditions. Representative Phil Gingrey, an obstetrician-gynecologist from Georgia who is co-chairman of the group, said the young-adult provision is "a good policy."

Insurers aren't promising to continue a provision of the law that requires them to cover children with pre-existing illnesses and said nothing about covering sick adults, required beginning in 2014. Those policies, called "guaranteed issue," can't be preserved unless the Supreme Court retains the requirement that Americans carry insurance, or Congress replaces it with some other measure compelling coverage, said Karen Ignagni, president and CEO of America's Health Insurance Plans, the industry's primary lobbying group in Washington.

Medicare Experiment

In addition to the insurance mandate, the law authorized Medicare to experiment with new ways to pay hospitals and doctors that would encourage higher quality, more efficient care. One program, called accountable-care organizations or ACOs, awards hospitals and physician groups a share of any savings they can produce by streamlining care and reducing unnecessary or wasteful services.

Striking down the law and eliminating Medicare's experiments in payment changes may harm insurers' efforts to slow cost growth if hospitals and doctors decide to resist them, said John Rother, president and CEO of the National Coalition on Health Care, a nonprofit in Washington that advocates for expanded coverage and reduced cost growth. Medicare is the government's health program for the elderly and disabled.

Presbyterian Healthcare Services, an eight-hospital chain in New Mexico, was one of the first participants in Medicare's experiment in accountable care. In the event the court wipes out the measure, Congress and the next president are likely to find themselves re-creating many of the same programs because of budget pressure, said Jim Hinton, Presbyterian's president.

"The basic need to rein in the cost of Medicare as a bellwether payer for the whole health-care system doesn't change," he said in a telephone interview.

Insurers, under pressure from employers to reduce health costs, have echoed many of Medicare's adaptations, such as the accountable care groups, Ignagni said.