And what makes it unconstitutional is its requirement that others states enter the agreement with NJ.
If NJ wanted to pass a law that elimiated voting by citizens for president and automatically instructed its electors to vote for the national popular vote winner that would be 100% constitutional.
Section 4
"The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence."
Disenfranchising the citizens of a state does not comply with the Constitution.