No. It's not unconstitutional. In the 1978 case of U.S. Steel Corporation v. Multistate Tax Commission, the compact at issue specified that it would come into

No. It's not unconstitutional. In the 1978 case of U.S. Steel Corporation v. Multistate Tax Commission, the compact at issue specified that it would come into force when seven or more states enacted it. The compact was silent as to the role of Congress. The compact was submitted to Congress for its consent. After encountering fierce political opposition from various business interests concerned about the more stringent tax audits anticipated under the compact, the compacting states proceeded with the implementation of the compact without congressional consent. U.S. Steel challenged the states' action. In upholding the constitutionality of the implementation of the compact by the states without congressional consent, the U.S. Supreme Court applied the interpretation of the Compacts Clause from its 1893 holding in Virginia v. Tennessee, writing that: "the test is whether the Compact enhances state power quaod [with regard to] the National Government." The Court also noted that the compact did not "authorize the member states to exercise any powers they could not exercise in its absence."