The Supremacy Clause

Under the Articles of Confederation, states often ignored national treaties and requests, and the framers sought a way to make federal law effective without giving Congress a veto over state laws, an idea James Madison favored but the Constitutional Convention rejected. Instead they adopted Article VI, Clause 2, which declares that the Constitution, the laws of the United States made in pursuance of it, and all treaties are the supreme law of the land, and that judges in every state are bound by them, anything in state constitutions or laws to the contrary notwithstanding.

The clause is the basis for federal preemption, under which valid federal law displaces conflicting state law, whether Congress says so expressly, occupies an entire field, or creates a conflict that makes compliance with both impossible. In McCulloch v. Maryland in 1819, the Supreme Court relied on it to strike down a state tax on the national bank, and in Cooper v. Aaron in 1958 it held that state officials are bound by the Court's interpretation of the Constitution, rejecting Arkansas's resistance to school desegregation.

The clause makes federal law supreme only within the federal government's constitutional powers, and the Court has held that Congress cannot commandeer state legislatures or officers to carry out federal programs, as in Printz v. United States in 1997 and Murphy v. NCAA in 2018. That principle underlies disputes over sanctuary policies, in which some states and cities decline to assist federal immigration enforcement. Supporters of broad preemption say national rules promote uniformity, while defenders of state authority say the Constitution leaves the states room to set their own policies.