Nullification Vs. Interposition
In theory, nullification differs from interposition in several respects. Nullification is usually considered to be an act by a state finding a federal law unconstitutional, and declaring it void and unenforceable in that state. A nullification act often makes it illegal to enforce the federal law in question. Nullification arguably may be undertaken by a single state.
Interposition also involves a declaration that a federal law is unconstitutional. There are various actions that a state might take to "interpose" once it has determined that a federal law is unconstitutional. In the Virginia Resolutions of 1798, Madison did not describe the form or effect of interposition. But two years later in the Report of 1800, Madison described a variety of actions that states might take to "interpose": communicating with other states about the unconstitutional federal law, attempting to enlist the support of other states, petitioning Congress to repeal the law, introducing Constitutional amendments in Congress, or calling a constitutional convention. Madison did not argue that a state could "interpose" by legally nullifying a federal law and declaring it unenforceable. Madison contemplated that interposition would be a joint action by a number of states, not an action by a single state. Interposition is considered to be less extreme than nullification because it does not involve a state's unilateral decision to prevent enforcement of federal law.
In practice, nullification and interposition often have been confused, and sometimes have been used indistinguishably. John C. Calhoun indicated that these terms were interchangeable, stating: "This right of interposition, thus solemnly asserted by the State of Virginia, be it called what it may — State-right, veto, nullification, or by any other name — I conceive to be the fundamental principle of our system." During the fight over integration of the schools in the south in the 1950s, a number of southern states passed so-called "Acts of Interposition" that actually would have had the effect of nullification.
As noted above, the courts have rejected both nullification and interposition.
Read more about this topic: Nullification (U.S. Constitution)