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 it’s not really a compact. It’s 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 that’s where it falls apart. The first time a popular vote goes against a state’s voters and that state’s EV’s would flip the election, the state will likely fail to abide, and there’s nothing any other state could do about it.