Although the Compact Clause of the U.S. Constitution (U.S. Const., Art. 1, Sec. 10, Clause 3)
appears to require congressional consent in every case (“No State shall, without the consent of
Congress…enter into agreement or compact with another State or with a foreign power…”) the
Supreme Court has determined that the clause is activated only by those agreements that would
alter the balance of political power between the states and the federal government or intrude on a
power reserved to Congress (Virginia v. Tennessee, 148 U.S. 503 (1893)). Therefore, where an
interstate agreement accomplishes nothing more than what the states are otherwise empowered to
do unilaterally, the compact does not intrude on federal interests requiring congressional consent.
Once congressional consent is granted and appropriate, the nature of the compact changes. It no
longer stands as an agreement between the states but is transformed into the “law of the United
States” under the law of the union doctrine (Cuyler v. Adams, 449 U.S. 433, 440 (1981).