If the public option became explicitly linked to Medicare — requiring all providers who wanted Medicare patients to accept public-option patients, too — the public option might be able to negotiate low prices for care and include a wide range of doctors and hospitals. In most markets of the country, that might make it far more appealing than the choices that people who buy their own insurance now have.

Insurers would have to adjust. Either they would also have to lower prices, or they would have to offer some sort of special services. Otherwise, they would lose a lot of customers. “It would push them to demonstrate the value of what they are selling,” said Linda Blumberg, a fellow at the Urban Institute. Her research has estimated the effects of several public-option plans, and found that the plans would tend to change remaining private insurance.

There’s also the possibility that linking public-option coverage to Medicare could cause some doctors to stop accepting Medicare patients, Ms. Glied said. That would be another form of politically risky disruption.