In May New Jersey imposed a health-insurance mandate requiring all residents to buy insurance or pay a penalty. More states will feel pressure to follow suit in the coming year as the federal mandate’s penalty disappears Jan. 1 and state legislatures reconvene, some with new Democratic majorities intent on “protecting” ObamaCare. But conflicts with federal law will make state-level health-insurance mandates ineffective or unduly onerous, and governors and legislatures would do well to steer clear.

While states can require...