I think that Wikipedia article is far too sanguine about the likelihood that NPVIC would pass Constitutional muster.
First, it's not likely that the wording of Article II truly grants state legislatures unlimited discretion in the selection of electors. A state law that required electors to be men or Methodists would surely be unconstitutional today. And as a result, there's almost certainly an equal protection argument to be made against NPVIC: Sure, each individual voter is participating equally in a larger process to choose the President, but that's not the process prescribed by the Constitution. The process demanded by the Constitution is a state-by-state selection of electors, and if a state legislature wants to have an election, it better be an election in which every voter of that state is participating equally. A thought experiment: Could the state of California pass a law that counted every vote for the electoral slate of the Democratic party twice? Surely not. So how can they pass a law which discounts the vote of every voter except those that voted for the nationwide popular winner?
Second, if this agreement doesn't trigger the Congressional approval requirement of the Compact clause of Article I, I can't imagine what would.
Yes, something like that note is more or less exactly what I'd expect SCOTUS' position to be. And I don't think it would be a close decision.
Then the question becomes, if Congress endorses the pact, could it go into effect? The success of an equal protection claim against the agreement is a little trickier to forecast, since the justices who typically support a broad reading of equal protection are likely to be the most sympathetic to arguments for eliminating the disparity between the popular vote and the EC. But I think the Court would see NPVIC, rightly, as an "end run" around the amendment process, and require that state elections remain state elections: States do not have the power to facilitate national or interstate plebiscites outside the usual Constitutional order.
First, it's not likely that the wording of Article II truly grants state legislatures unlimited discretion in the selection of electors. A state law that required electors to be men or Methodists would surely be unconstitutional today. And as a result, there's almost certainly an equal protection argument to be made against NPVIC: Sure, each individual voter is participating equally in a larger process to choose the President, but that's not the process prescribed by the Constitution. The process demanded by the Constitution is a state-by-state selection of electors, and if a state legislature wants to have an election, it better be an election in which every voter of that state is participating equally. A thought experiment: Could the state of California pass a law that counted every vote for the electoral slate of the Democratic party twice? Surely not. So how can they pass a law which discounts the vote of every voter except those that voted for the nationwide popular winner?
Second, if this agreement doesn't trigger the Congressional approval requirement of the Compact clause of Article I, I can't imagine what would.