Federal Preemption

Article VI makes the Constitution, federal laws, and treaties the supreme law of the land, binding on state judges notwithstanding anything in state law. When Congress acts within its powers, conflicting state law gives way. Chief Justice John Marshall applied the principle in Gibbons v. Ogden in 1824, striking down a New York steamboat monopoly that conflicted with a federal coastal license.

Courts recognize three kinds of preemption. Express preemption occurs when a federal statute says it displaces state law, as the law governing employee benefit plans does broadly. Field preemption occurs when federal regulation is so pervasive, as with nuclear safety or immigration registration, that Congress left no room for states. Conflict preemption occurs when it is impossible to obey both laws or state law stands as an obstacle to federal goals. In Arizona v. United States in 2012 the Supreme Court struck down most of an Arizona immigration law on preemption grounds. Courts begin with a presumption against preemption in areas traditionally regulated by states.

Preemption fights shape policy in nearly every field, from drug labeling and auto emissions to banking, data privacy, and artificial intelligence. Businesses often favor federal preemption to avoid fifty different rules, while states defend their authority to set stronger protections. In 2025 the Senate voted 99 to 1 to strip from the budget bill a proposed moratorium on state AI laws, a sign of how contested the question remains. State marijuana legalization, in tension with federal law that still prohibits the drug, shows that federal law can coexist with conflicting state policy when federal enforcement is limited.