Interposition is an asserted right of U.S. states to declare federal actions unconstitutional. Interposition has not been upheld by the courts. Rather, the courts have held that the power to declare federal laws unconstitutional lies with the federal judiciary, not with the states. The courts have held that interposition is not a valid constitutional doctrine.

Interposition is closely related to the theory of nullification, which holds that the states have the right to nullify federal laws that are deemed unconstitutional and to prevent enforcement of such laws within their borders.

Though interposition and nullification are similar, there are some differences. Nullification is an act of an individual state, while interposition is undertaken by states acting jointly. Nullification is a declaration that a federal law is unconstitutional accompanied by a declaration that the law is void and may not be enforced in the state. Interposition also involves a declaration that a federal law is unconstitutional, but after the interposition that law would still be enforced in the state.

There are various actions that a state might take to "interpose" once it has determined that a federal law is unconstitutional, including communicating with other states about the unconstitutional 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. In this sense, interposition could be seen as more moderate than nullification (and less likely to result in federal retaliation).

Interposition and nullification often are discussed together, and many of the same principles apply to both theories. In practice, the terms nullification and interposition often 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 desegregation of the schools in the south in the 1950s, a number of southern states tried to preserve their segregated schools by passing so-called "Acts of Interposition" that actually had the effect of nullification. These acts were stricken down by the courts, whether labelled acts of interposition or nullification.

Read more about Interposition:  The Constitution and The Theory of Interposition, Virginia Resolution, Interposition Attempts in The 19th Century, School Desegregation, Contemporary Debate

Other articles related to "interposition":

Nullification (U.S. Constitution) - Nullification Attempts and School Desegregation in The 1950s
... Nullification and interposition resurfaced in the 1950s as southern states attempted to preserve racial segregation in their schools ... At least ten southern states passed nullification or interposition measures attempting to preserve segregated schools and refusing to follow the Brown decision ... The advocates of these nullification and interposition measures argued that the Brown decision was an unconstitutional infringement on states' rights, and that the states had the power to ...
Interposition - Contemporary Debate
... Interposition and nullification have been raised recently in several state legislatures ... Interposition or nullification bills have been introduced in several state legislatures ... Opponents respond that interposition is not a valid constitutional doctrine and has been discredited ...
Nullification (U.S. Constitution) - Nullification Vs. Interposition
... In theory, nullification differs from interposition in several respects ... Interposition also involves a declaration that a federal law is unconstitutional ... In the Virginia Resolutions of 1798, Madison did not describe the form or effect of interposition ...
Kentucky And Virginia Resolutions - Influence of The Resolutions - School Desegregation
... Kilpatrick, relying on the Virginia Resolution, revived the idea of interposition by the states as a constitutional basis for resisting federal government action ... Virginia, and Florida, subsequently passed interposition and nullification laws in an effort to prevent integration of their schools ... Aaron, the Supreme Court unanimously rejected Arkansas' effort to use nullification and interposition ...
LeRoy Collins - Governorship
... attempt to prevent them from passing an "interposition" resolution, which indicated the intent of the legislature to "interpose" itself between the citizens of Florida and the United States ... However, as it passed through his office, he wrote upon the interposition resolution, the following statement, in his own handwriting, which is today held ... Not only will I not condone 'interposition' as so many have sought me to do, I decry it as an evil thing, whipped up by the demagogues and carried on the hot and erratic winds of passion, prejudice ...