Interposition
Interposition is a claimed right of a U.S. state to oppose actions of the federal government that the state deems unconstitutional. Under the theory of interposition, a state assumes the right to "interpose" itself between the federal government and the people of the state by taking action to prevent the federal government from enforcing laws that the state considers unconstitutional. In Cooper v. Aaron (1958) the Supreme Court of the United States rejected interjection explicitly. It and the lower federal courts have consistently 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 when invoked to block enforcement of federal law.