The part of the law that lets people stay on their parents' insurance until age 26, one of the first provisions enacted, led 6.6 million young adults to join those plans in 2011, according to the Commonwealth Fund, a New York-based nonprofit that conducted the survey and supports expanded coverage.

The insurers won't be obligated to stick by their pledges if the law is rejected, and large employers who insure themselves and only hire companies such as UnitedHealth to administer benefits for their workers could refuse to pay for the extra benefits.

"We can't make anyone buy anything," said Matt Stearns, a spokesman for UnitedHealth, in an e-mail. He said the company isn't able to say how much the provisions may add to premiums.

Clearest Evidence

The statements by the insurers are the clearest evidence that measures of the law that have already been implemented may outlive the statute itself, said David Cutler, an economics professor at Harvard University who helped policy makers draw up the Massachusetts health-care law that formed the basis for President Barack Obama's national policy.

"You can take away the law, but you can't take away what's gone on since," Cutler, who advised Obama's presidential campaign in 2008, said in an interview in Washington. "The idea that you're going to wipe away Obamacare is both not practical and not even what anyone wants to do."

Humana, which has 5.9 million customers in its commercial health plans, said in a statement yesterday that its customers "should have the peace of mind of knowing the company embraces and will maintain these common-sense provisions that add stability and security to health coverage."

Future Decisions

The Blue Cross Blue Shield Association, whose 38 local member companies insure about 100 million people, will encourage the plans "to offer their customers the broadest set of protections possible at an affordable price," said Kelly Miller, a spokeswoman.

WellPoint Inc., the second-largest insurer by market value and owner of Blue Cross Blue Shield plans in 14 states, will announce specific coverage plans after the court ruling, said Kristin Binns, a spokeswoman for the Indianapolis-based company.