Nullification (U.S. Constitution)
Nullification, in United States constitutional history, is a legal theory that a state has the right to nullify, or invalidate, any federal law which that state has deemed unconstitutional. The theory of nullification has never been legally upheld; rather, the Supreme Court has rejected it.
The theory of nullification is based on a view that the States formed the Union by an agreement (or "compact") among the States, and that as creators of the federal government, the States have the final authority to determine the limits of the power of that government. Under this, the compact theory, the States and not the federal courts are the ultimate interpreters of the extent of the federal government's power. Under this theory, the States therefore may reject, or nullify, federal laws that the States believe are beyond the federal government's constitutional powers. The related idea of interposition is a theory that a state has the right and the duty to "interpose" itself when the federal government enacts laws that the state believes to be unconstitutional. Thomas Jefferson and James Madison set forth the theories of nullification and interposition in the Kentucky and Virginia Resolutions in 1798. A more extreme assertion of state sovereignty is the related action of secession, by which a state terminates its political affiliation with the Union.
The theory of nullification has been rejected repeatedly by the courts. The courts have found that under the Supremacy Clause of the Constitution, federal law is superior to state law, and that under Article III of the Constitution, the federal judiciary has the final power to interpret the Constitution. Therefore, the power to make final decisions about the constitutionality of federal laws lies with the federal courts, not the states, and the states do not have the power to nullify federal laws.
Between 1798 and the beginning of the Civil War in 1861, several states threatened or attempted nullification of various federal laws, including the Supreme Court of Wisconsin's ruling in 1854 that the Fugitive Slave Act of 1850 was unconstitutional. None of these efforts were legally upheld. The Kentucky and Virginia Resolutions were rejected by the other states. The Supreme Court rejected nullification attempts in a series of decisions in the 19th century, including Ableman v. Booth, which found that Wisconsin did not have the power to nullify the Fugitive Slave Act. The Civil War ended most nullification efforts.
In the 1950s, southern states attempted to use nullification and interposition to prevent integration of their schools. These attempts failed when the Supreme Court again rejected nullification in Cooper v. Aaron, explicitly holding that the states may not nullify federal law.
Read more about Nullification (U.S. Constitution): The Kentucky and Virginia Resolutions, Nullification Attempts and School Desegregation in The 1950s, Nullification Vs. Interposition, Nullification Compared To Other Actions By The States