[Cletus.D.Yokel]

But, if they based the appointments on the result from any others States and according to a previous agreement, that is an interstate pact (treaty) subject to Congressional approval.

[WaltStuart]

I think that’s a hugely valid point about interstate compact/treaty etc.

However, functionally, therefore what? Would go to the Supreme Court? What are the odds then?

[Publius]

If we're talking about the National Popular Vote Compact (NPV), there are two factors to weigh.

1. Congress must approve all interstate compacts. The proponents of NPV claim that this would not apply to the NPV because of the plenary power of state legislatures to choose electors. They may be right. This is something the Supreme Court would have to untangle.
2. Article IV, Section 4 of the Constitution guarantees a republican form of government to the states. Is it "republican" to assign electoral votes to a candidate who did not win the state's popular vote? Again, we run into the plenary power of state legislatures being the final authority to assign presidential electors. The Supreme Court would have to weigh in to untangle this mess.

If Trump wins in a landslide of the popular vote, the NPV will quietly disappear.