Final answer:
Nullification is the concept that a state may nullify federal laws it deems unconstitutional. It was notably used during the Nullification Crisis over tariff acts in the 1830s and is tied to states' rights versus the authority of the federal government.
Step-by-step explanation:
Nullification is the concept that a state may nullify or refuse to obey or enforce any federal law it considers unconstitutional. Interposition, related to nullification, involves states blocking federal government laws and actions they deem to exceed the constitutional powers of the national government. However, the U.S. courts have been traditionally unsympathetic to such arguments, with the exception being when the federal government directly requires state and local officials to undertake certain actions.
Instances such as the 1997 Supreme Court striking down a part of a federal law requiring state and local law enforcement to conduct gun purchase background checks, and the 2012 ruling that states cannot be compelled to participate in Medicaid expansion, exemplify the courts' stance. The concept has also been historically linked to the defense of slavery, as evidenced during the Nullification Crisis in the 1830s over the tariff acts. Led by John Calhoun, nullifiers like South Carolina passed an Ordinance of Nullification but eventually backed off due to the Force Bill and the Compromise Tariff Act of 1833.