However, it is Unconstitutional as it is in DIRECT violation of the Interstate Compact provision in Article 1 Section 10 of the Constitution: No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
The trouble is its not really a compact. Its a state passing a law whose enforcement is contingent on other states actions. There is no direct agreement among the states involved; there are only individual state laws that take effect conditionally. If it were a compact, then states abiding by the compact could sue a state that agreed to it but later broke its terms. I think thats where it falls apart. The first time a popular vote goes against a states voters and that states EVs would flip the election, the state will likely fail to abide, and theres nothing any other state could do about it.