Nullification is the idea that a state can invalidate or refuse to enforce a federal law that it believes is unconstitutional. This concept emerged in the early 19th century, particularly during the Nullification Crisis in the 1830s, when South Carolina attempted to nullify federal tariffs. The crisis highlighted tensions between state and federal authority. The theory of nullification is rooted in the belief that the U.S. Constitution is a compact among states, allowing them to challenge federal overreach. However, the idea has been largely rejected by the courts and is not widely accepted in modern legal interpretations, reinforcing the supremacy of federal law.