[Gumption]

If you're referring to Article 4, Section 4 ...

The United States shall guarantee to every State in this Union a Republican Form of Government,

I agree with you.

[Bob]

There's also Article 1, Section 10:

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...

Such an agreement between states needs to be approved by Congress.