Some health care experts have suggested that there might be ways to replace the insurance mandate by using market incentives to encourage the uninsured to gain coverage.

The government could, for instance, give consumers limited enrollment periods to buy insurance so they could not wait until they were ill. Or it could tell them that if they did not buy a policy during a defined period they also would not be eligible for benefits, like subsidized coverage.

But supporters of the health care law contend that such measures would insure only a fraction of the 30 million estimated to gain coverage with the mandate in place.

“There are alternatives to the mandate, but none of them are considered as effective,” said Neera Tanden, a former Obama aide who now heads the Center for American Progress.