But audaciously, the states argued — and Judge O’Connor agreed — that the rest of Obamacare must fall, too. They claim that the mandate is so central to the A.C.A. that nothing else in it can operate without it.

That’s not how the relevant law works. An established legal principle called “severability” is triggered when a court must consider what happens to a statute when one part of it is struck down. The principle presumes that, out of respect for the separation of powers, courts will leave the rest of the statute standing unless Congress makes clear it did not intend for the law to exist without the challenged provision. This is not a liberal principle or a conservative principle. It is an uncontroversial rule that every Supreme Court justice in modern history has applied.

Sometimes severability cases are difficult because it is hard to guess how much importance Congress attributed to one provision, especially in a lengthy law like the Affordable Care Act. But this is an easy case: It was Congress, not a court, that eliminated the mandate penalty and left the rest of the statute in place. How can a court conclude that Congress never intended the rest of the statute to exist without an operational mandate, when it was the 2017 Congress itself that decided it was fine to eliminate the penalty and leave the rest of the law intact?