Isn't that a direct violation of the 14th amendment? Specifically, Section 2.
I don't think so. That section says, "But when the right to vote at any election for the choice of electors for President and Vice-President of the United States, Representatives in Congress, the Executive and Judicial officers of a State, or the members of the Legislature thereof, is denied to any of the male inhabitants of such State, being twenty-one years of age, and citizens of the United States, or in any way abridged, except for participation in rebellion, or other crime, the basis of representation therein shall be reduced in the proportion which the number of such male citizens shall bear to the whole number of male citizens twenty-one years of age in such State."
The right to vote isn't abridged by NPV; the results of the elections are ignored. That could be considered abridging the right to vote, but if so it would apply to the entirety of the electorate and would therefore reduce the representation in Congress by the proportion of those who voted in the election to the entire voting-age population of the state. The states that did this would surely howl if their Congressional representation was reduced by roughly 60% (it would be fun to watch), but I doubt it would ever get that far. NPV would be declared unconstitutional for the other reasons first, I should think.