For insurance companies nervously watching the legal fight over the constitutionality of the President’s health-care law, it would be the unthinkable: The U.S. Supreme Court strikes down the law’s so-called individual mandate, which requires millions of young, healthy people to buy coverage—but leaves intact rules compelling insurers to cover sick people, who are likely to cost far more in benefits than they pay in premiums. Congress is then left to fix the problem. The scenario is a real possibility. In a challenge to the health-care law brought by 26 states, a federal appeals court took exactly that approach, declaring in August that the mandate is unconstitutional while saying the rest of the law could stand. The Supreme Court is expected to take up the question of the statute’s constitutionality next year.