In the first Affordable Care Act case three years ago, the Supreme Court had to decide whether Congress had the power, under the Commerce Clause or some other source of authority, to require individuals to buy health insurance. It was a question that went directly to the structure of American government and the allocation of power within the federal system.

The court very nearly got the answer wrong with an exceedingly narrow reading of Congress’s commerce power. As everyone remembers, Chief Justice John G. Roberts Jr., himself a member of the anti-Commerce Clause five, saved the day by declaring that the penalty for not complying with the individual mandate was actually a tax, properly imposed under Congress’s tax power.