In McPherson v. Blacker 1892], the Court ruled that the state has a plenary [meaning exclusive, unalterable] right to establish the method of selecting its electors and the Compact Clause does not apply ...
However, the condition where a state grants the right to vote to its citizens [example: winner-take-all] , but then REVERSES the outcome in the state [and awards its electoral votes to the winner of the national popular vote] WAS NOT examined in McPherson v. Blacker 1892] ...
In Bush v. Gore [2000], the Court ruled that once the right to vote to its citizens [example: winner-take-all]:
The right to vote is protected in more than the initial allocation of the franchise. Equal protection applies as well to the manner of its exercise. Having once granted the right to vote on equal terms, the State may not, by later arbitrary and disparate treatment, value one person's vote over that of another. See, e.g., Harper v. Virginia Bd. of Elections, 383 U.S. 663, 665 (1966) (“[O]nce the franchise is granted to the electorate, lines may not be drawn which are inconsistent with the Equal Protection Clause of the Fourteenth Amendment”). It must be remembered that “the right of suffrage can be denied by a debasement or dilution of the weight of a citizen’s vote just as effectively as by wholly prohibiting the free exercise of the franchise.” Reynolds v. Sims, 377 U.S. 533, 555 (1964).
According to Bush v. Gore, under the NPV Compact, the state would be favoring the national popular vote of ALL the other states [plus the minority votes in its state] over the majority of voters in its state that voted for the other guy/gal ...
These two rulings are diametrically opposed to each other. If the NPV Compact Law was ever invoked by a state in an election [overturning the inital allocation of its electoral voters to one candidate and handing them to the winner of the NPV], then the Court would have to decide whether the state's plenary right to choose its electors in the manner that it sees fit SUPERCEDES the individuals right to Equal Protection under the 14th Amendment ...
However, the current NPV movement is such that the individual state's method of choosing its electors only changes when enough states join the compact totaling 270 electoral votes.
As I've said before in past threads, if a state chooses to award its electoral votes to the winner of the national popular vote REGARDLESS of how many other states join the compact, more power to them -- it's their plenary right as you indicate. Those states should begin doing so in 2012 without waiting for other states.
It is only when they make it contingent on other states joining the compact that things change. At that point, in my opinion it becomes a compact between states requiring Congressional approval.
-PJ