(1) We don't have any mechanism for getting information about a national popular vote; the add-all-the-state-numbers-together tally is a complete fiction that has no direct meaning. The United States under the Electoral College does not run one election; it runs fifty-one different elections (states plus D.C.), weights them by population, and gives the laurels to the person who won enough elections of sufficient weight. Each of these elections is run on different laws governing means of collecting votes, times and places, means of counting, and even who can vote and how. Adding numbers from different elections doesn't get you a 'national popular vote' number, because they are measuring different things. What is more, there is always a certain amount of uncertainty in elections involving large populations (and the U.S. electorate is very large); we have no mechanisms, as we would need if we were collecting a real national popular vote number, for minimizing this uncertainty, and, indeed, trying to guesstimate a national popular vote number from the elections we do have necessarily multiplies the uncertainty.
(2) The 'National Popular Vote Compact' is not a national popular vote system; the name is a lie. It is an Electoral College system under which states agree to ignore the decisions of their own populations and distribute their Electoral College votes based on a number that was obtained inconsistently with their own election laws and by methods that they cannot themselves properly monitor and correct. It is indeed as stupid as that sounds. As I have said before:
On the NPV system, states would be committing themselves in the Electoral College to preferring votes elsewhere to those cast by their own citizens. If State A doesn't allow felons to vote and State B does for civil rights reasons, then on the NPV plan, State A is committed to accepting as legitimate felons voting in in State B despite the fact that people in A exactly like those in B don't get to vote, and State B is committed to accepting as legitimate the election numbers coming out of State A, despite knowing quite well that the numbers are derived in part on what people in State B regard as a civil rights violation, and that there are potential voters in A whose votes are not getting counted despite the fact that they would count in B. This is an absurd situation. Moreover, NPV guarantees that states with well-thought-out election laws and well-run election systems are held hostage to those without....Numbers can't be established for a 'national popular vote' (even one based on a fiction) under a state-by-state system like ours unless all the states have their act together. We know for a fact that this can't be guaranteed, and that a state can make a complete mess of things by poor collection methods, inconsistent vote-counting, and loopholes for voting fraud. And we also know for a fact that nobody can actually fix these problems except citizens of that state.
Any state legislature that is so stupid as to sign on to the Compact is failing in their responsibilities to their own citizens.
(3) Because it is not a real national popular vote, and involves nothing remotely like what would be required for a real national popular vote, no arguments for a national popular vote actually give one a reason for supporting the National Popular Vote Compact. And because it doesn't have any mechanism for guaranteeing equal votes, no argument for equalizing votes can give a reason for supporting the National Popular Vote Compact. And because it is an Electoral College system that is designed on principles inconsistent with the Electoral College itself, no arguments about how the Electoral College could better represent the people of the United States can possibly give a reason for supporting the National Popular Vote Compact, either. There is no good reason for it. The Electoral College is resilient enough that maybe -- maybe -- it could avoid disaster, but a proposal that is so incoherent -- and it is, again, literally incoherent -- cannot possibly be good for an electoral system.
(4) The proposal depends on an attack on the integrity of the Electoral College; it requires claiming that the Electoral College as it is intended to function is not getting good results. But at the same time, the proposal does not eliminate the Electoral College, and, indeed, the entire point of the proposal is to avoid going through the proper process to amend the Constitution. This is a further incoherence in the plan: it is, and this is often explicit in the defenses of the defenders, an attempt to treat a provision of the Constitution as defective while simultaneously pretending it doesn't need actual correction. Any citizen in any state should regard a legislator's vote for the NPV plan as an act of contempt for the United States Constitution and as a sign of incompetence, because it takes both stupidity and contempt for the Constitution to treat such a ridiculous proposal as a serious election system.