I believe that NPV can be challenged before the US Supreme Court, and that it can be defeated there.
There are those here who will observe (correctly) that the Constitution "leaves it up to the states" as to how electors will be assigned.
And that is [largely] correct. Seems to me that very early in the nation's history, didn't a few of the states assign electors without conducting a "presidential election"? If they did, well... that was in compliance with the Constitution. At least it was... back then.
However, today all 50 states conduct popular elections in which the voters choose the electors. In most states all electors are assigned to whomever wins the popular vote. In two states (Maine and Nebraska), the electors are assigned proportionally by the winners of the Congressional district involved. This is how Mr. Trump won one elector from Maine in 2016.
But... NPV seeks to change things, by permitting states to essentially overturn the popular vote of elections conducted within their own borders and arbitrarily assign their electors based on the "popular vote" of OTHER states.
And this is how it can be beaten.
That is to say... (and I'll use Nevada as my example)
...If Nevada wishes to adopt NPV, they then have the Constitutional right to assign their electors to the winner of the popular vote in OTHER states.
BUT... they must do so WITHOUT HOLDING AN ELECTION FOR PRESIDENTIAL ELECTORS IN NEVADA.
If they DO choose to have an election by which the voters will choose the electors, then they must ABIDE THE VOTE of the citizens of Nevada.
To do otherwise will violate numerous laws and Constitutional principles, including the 14th Amendment (which guarantees equal protection of the laws to all), and certainly the Voting Rights Act (because the votes cast by individual voters will be discarded in favor of the votes from out-of-staters, effectively disenfranchising the voters of Nevada).
I believe the Supreme Court (at least a modestly conservative one, which we have now) would rule that way. To wit:
- Assign electors however you wish, BUT...
- If you do have an election for president, you must assign electors based upon that vote.
So... California and New York could choose NOT to have an election if they wanted to comply with NPV. But what will "the absence" of the popular vote in CA and NY do to the "national popular vote total"???
Having said all this...
I can see leftist states then proclaiming, OK, we're NOT going to have an election that selects electors, but we'll have a "popularity vote" within the state instead. Attempting to end run a Court decision as outlined above.
Then, it would be up to the Supreme Court to once more decide if this is permissible.
If they decide in favor of the leftist states, it will become the "Dred Scott" decision of the twenty-first century. That is to say, it may become "the fuse" that ignites a revolt of the less-populated red states, which will have lost one of the "great balancers" established by the Constitution to give them equity against the large states. What will they then have left to lose?