But in the vast majority of states that rely on the federal exchange, HealthCare.gov, there is little or no evidence that anyone has a plan to preserve the subsidies that help more than six million residents of those states afford their insurance premiums. Most of the affected states have Republican governors, and many, including Scott Walker of Wisconsin, Rick Scott of Florida and Dennis Daugaard of South Dakota, insist it is Congress’s job to come up with a remedy if the subsidies disappear.

Michael O. Leavitt, who was secretary of health and human services under President George W. Bush, tried to nudge more federal exchange states into action in a letter to their governors and legislators last week.

“They’re not scrambling as of yet over this,” Mr. Leavitt said. “But when the force of millions of people who are going to have their insurance affected begins to influence this debate, it’s going to look different to those who are feeling the pressure. And I think it will be the governors.”