For comparison, note that a lot of what we think of as "treaties" (which require advice and consent of the Senate) aren't actually treaties per the constitutional interpretation, but done only with executive privilege.
States just need to pass a law which says: "If greater than [threshold] of states pass NPVIC-compliant laws, we will select electors based on popular vote of America, rather than on popular vote of [state]."
This type of collusion very obviously doesn't meet the threshold under Constitutional terms, and continues to be a republican form of government. I don't see any issues.
This sort of collusion is pretty common too. See local COVID19 response state consortia, codes like UCC, professional accreditations, etc. Without that, the US government would be impotent, since there are things which are outside of federal power, but which require coordination across states. If this were banned, the US would collapse.
Yes, with:
> Toss out the states votes?
...as a fallback.
That's actually how the norm of "have a vote of the people in your state with the actual voting on, or completed by, the standard 'election day', and assign electors by a procedure that exists in law prior to that election" rule works. Its a federal law directing what states should do, and threatening that their electoral votes might be disregarded when the electoral vote count occurs in Congress if they don't comply.
As long as the defecting state had fewer (or the same number of) electoral votes as the aggregate of the non-compacting states that happened to assign their electors in support of the popular vote winner, then the defection would not actually change the outcome of the election.
This may reduce the incentive for states to defect from the compact, as might the possibility of electors from non-compacting states becoming faithless out of respect for the compact.
Strategically I think the NPVIC shouldn't have been presented as a compact between states but as a set of individual pledges to the people of America that each pledging state would act in the necessary way once enough other states had made equivalent pledges. That would mean the states wouldn't have standing to sue each other, unlike in a compact, but it seems that their ability to do that is constitutionally questionable anyway.
I'm pretty sure that in the cases around the 2000 Presidential election (and possibly previously) one of the key federal rulings is that because selection of Presidential electors is a power of state legislatures (not states as a whole) specifically granted by the federal Constitution, State Constitutions cannot trump the federal Constitution in how that authority is allocated.
I would disagree with that phrasing, and your final paragraph contradicts that statement. There is nothing unconstitutional about states entering into compacts. They merely require congressional consent to do so as the language of Article 1 Section 10 makes clear.
Additionally, even without congressional consent the supreme court has ruled multiple times since the late 1800s that as long as an a compact is not:
> "directed to the formation of any combination tending to the increase of political power in the States, which may encroach upon. . . . the just supremacy of the United States"
Then congressional consent is not required. [1][2][3]
The argument then is whether the NPVIC violates that rule and encroaches on federal supremacy or grants additional power to the states. However, the power in question here relates to how states appoint electors. This is a power explicitly granted to the states under article 2 section 1 clause 2 [4]:
> Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors
Therefore, in my opinion, any compact regarding the power to select electors does not pass the tests laid out by previous case law, and the compact does not require congressional approval. Of course our opinions don't count, only the supreme courts opinion matters, but there is no indication from any case law that this compact is "explicitly unconstitutional" or "in a certain way illegal". If anything it is borderline, but supported by current caselaw. The supreme court will most likely take up any suit involving the compact for clarification, but they would need a novel justification for deciding that states exercising a power explicitly granted to the states somehow increases the power of states.
Additionally, all of this is entirely moot if congress grants approval.
For further reading I would look at the NPVIC's own FAQ which has detailed citations and caselaw supporting their argument as well as detailing counter arguments [5]. For an independent review of interstate compact caselaw see [1], [6], or [7].
[1] https://ballotpedia.org/Interstate_compact
[2] https://ballotpedia.org/Virginia_v._Tennessee_(1893)
[3] https://ballotpedia.org/New_Hampshire_v._Maine_(1976)
[4] https://en.wikipedia.org/wiki/Article_Two_of_the_United_Stat...
[5] https://www.nationalpopularvote.com/section_9.16#myth_9.16.5
[6] https://www.csg.org/knowledgecenter/docs/ncic/caselaw.pdf
The problem is that this agreement is expressly unconstitutional by way of article 1 section 10: No State shall, without the Consent of Congress, [...] enter into any Agreement or Compact with another State
While states are allowed to divvy up their EVs within their own borders however they choose, this kind of interstate agreement to do so in a certain way is illegal, and bipartisan congressional support to circumvent the electoral college seems unlikely.