Article I, Section 10, Clause 3:

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.

For the most part the Supreme Court has let states create agreements on smaller things that wouldn’t be Congressional issues. This is a Congressional issue because they certify the vote so a politically biased Supreme Court is nearly certain to shut it down. Right now the National Popular Vote Interstate Compact isn’t in effect, so no one has standing to bring a case. But once it is in effect, expect a case to be quickly brought because opponents have had years to plan for this.