There are arguments for both views. Realistically given the political environment I have a feeling that the Supreme Court would decide that they do get to weigh in, and they would probably rule that congress has to approve it. In general, agreements between states, like the voting compact, do require congressional approval because having the feds as a mediator could prevent two states from getting into it or going to war with each other over any given compact. States are like tiny independent countries up until they arent, and this is one area where its sensible for keeping the peace if they arent allowed to be such a high degree of independent. Just like how some random state cant decide to have their own independent relationship with a foreign government

The good thing is that if that happens, then itll be cool beans either way.