This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any thing in the Constitution or Laws of any State to the Contrary notwithstanding.
The language of the supremacy clause has been construed to make all federal laws and treaties the supreme law of the land, taking priority over any state or local statutes or ordinances that conflict with federal law. Accordingly, state courts and state legislatures are bound by federal law and may not enact laws or make rulings contrary to federal law. Furthermore, state constitutions are not enforceable to the extent that they contradict federal law.
When enacted in accord with the powers granted in the U.S. Constitution, federal law will always prevail. However, any statute or treaty enacted or put in place by the federal government must be permissible under the U.S. Constitution; in other words, the right of Congress to enact any law must be “enumerated” or granted somewhere in the Constitution. For example, some of the enumerated powers of Congress include the power to pass laws relating to the military, immigration, taxation, and relations with foreign nations. Under the 10th Amendment to the U.S. Constitution, any powers not delegated to the federal government are held by the states and by the people, which means that state law governs a large number of topics. Congress generally may not get involved in state law matters like divorce and other family matters, personal injury cases, most real estate matters, and many criminal matters.
Furthermore, to be valid, a federal law must not infringe upon or violate specific limitations on federal powers set forth in the Constitution, including the Bill of Rights. So, for example, a federal law cannot be enforced if it violates the right of free speech or discriminates on the basis of race or sex.
The legal principle used to carry out the Supremacy Clause is commonly referred to as “federal preemption.” When ruling on whether a particular federal law takes precedence over a state law, a court may find that the federal law “preempts” state law and that the state law is therefore unenforceable.
The first Supreme Court case to interpret the Supremacy Clause came early in the nation’s history, in the case of McCulloch v. Maryland, in 1819. The Supreme Court ruled that the supremacy clause prohibited the application of state laws that would have allowed a federally chartered bank to be taxed by the state.
When the Supreme Court found, in 2015, that state prohibition of same-sex marriages is unconstitutional, the ruling effectively preempted all state laws that banned the practice. The supremacy clause has also been used to compel states to follow guidelines established by the federal Food and Drug Administration (FDA). Additionally, because there are federal drug laws that criminalize marijuana, the federal government could preempt state laws that legalize its possession and use. To date, though, the federal government has mostly declined to enforce the federal laws that criminalize marijuana use.