Preemption is the operational doctrine. Congress may expressly displace state law, occupy a regulatory field, or create a conflict in which simultaneous compliance is impossible or state law obstructs the federal purpose. Courts begin with statutory text and structure rather than assuming that any federal presence eliminates state authority.

A state may regulate alongside federal law when Congress permits complementary rules. It may not nullify a valid federal statute or impose requirements incompatible with it. Separate the constitutional question—could Congress legislate?—from the preemption question—what state law did the valid statute displace?

Supremacy resolves a conflict between valid rules; it does not rank governments for every purpose. A federal agency preference without legal authority cannot preempt state law, while a properly authorized federal standard can. Identify the controlling federal text and the exact conflict before choosing preemption.

Video lesson: Federalism: Crash Course Government and Politics #4