Specifically:

>No State shall, without the Consent of Congress, ... , enter into any Agreement or Compact with another State

In this case, since such a deal is changing how the president is elected, and changing the balance of power between states and the fed, it could probably be seen as encroaching on federal power. So simply getting congress to pass a resolution approving it (which in theory would solve the "consent of congress") wouldn't fly like it would with an interstate compact dealing with something more mundane and less political like water rights, or mutual deal on air pollution between several neighboring states.

Disclaimer: IANAL, this is not legal advice etc.

2) the part that says no interstate compacts without express approval of congress