"Very bad news for constitutional federalism."

Says Ilya Somin about today's decision in Comstock...

The big problem is not just that the Court ruled that Congress had the power to detain “sexually dangerous” federal prisoners who have already completed their sentences. By itself, this is a relatively minor policy (except, of course, for the people detained). The really dangerous element of the majority opinion is that it adopts the highly deferential “rational basis” test for assessing assertions of power under the Necessary and Proper Clause, holding that “in determining whether the Necessary and Proper Clause grants Congress the legislative authority to enact a particular federal statute, we look to see whether the statute constitutes a means that is rationally related to the implementation of a constitutionally enumerated power.”

Though "the statute involved here is somewhat peculiar,"  so that it might be distinguished in future cases, Eugene Volokh says that the Chief Justice's joining the majority suggests that the Court has lost its taste for striking down federal laws on the ground that they exceed Congress's enumerated powers.

To be sure, the facts of this case are unusual, because the law here applies only to people who had already been convicted of federal crimes. But the majority’s rationale seems quite broad; it concludes that federal power challenges should be upheld so long as they are “rationally related to the implementation of a constitutionally enumerated power” — that’s the famously extremely deferential “rational basis” test....

But, "the case has little or no import for the constitutional challenges to the individual health insurance mandate," says Randy Barnett:

Comstock involved whether ample connection existed between the law incarcerating sexual predators after their federal criminal sentence had been completed and an enumerated power....

With the challenges to the individual mandate, however, Congress is explicitly asserting that the individual mandate is “necessary and proper” to execute its power under the Commerce Clause. Moreover, the argument for “necessity” is reasonably straight-forward: it is necessary to compel all uninsured persons into the insurance pool to pay for the increased costs being imposed on insurance companies by the Act. Under the Court’s normal deferential approach, finding “necessity” won’t be hard.

The problem with the mandate is whether it is a “proper” means to achieve a constitutional end. ...

In Comstock, nothing about the incarceration of sexually dangerous persons was alleged to be an “improper” means of pursuing an enumerated end. The issue was whether or not the statute was enacted pursuant to an enumerated power....