In the Supreme Court last week, oral arguments in a case called Bruesewitz v. Wyeth turned on the meaning of the word “unavoidable,” but the real issue was something much bigger. The case is about whether the family of a girl, who they said was badly injured by a vaccine, can sue the manufacturer in a state court. Or are they barred by a 24-year-old federal law that blocks this sort of lawsuit if “the injury or death resulted from side effects that were unavoidable?” 

That prohibition is called pre-emption, a hot issue in constitutional law because it is at the heart of the balance of power between states and the federal government — the meaning of federalism. At stake is the ability of states to protect their citizens, by regulating health, safety, the environment, and other primary interests and by giving victims of wrongdoing redress in court, as long as a state law doesn’t conflict with a federal law....