Wikipedia has quite a good overview[0] of the constitutionality issues.
It may be that the NPVIC has to be replaced by an informal agreement, where the states that support the Compact all simultaneously but "unilaterally" change their system for appointing electors to one that implements a national popular vote. The trigger for this would that enough states have passed motions of intent to support this change "at some point". That would seem like no more of a Compact than one state lowering its sales tax when it notices a nearby state doing the same.
Hmm, this is interesting: As part of concerns about whether the NPVIC would shift power from the federal government to state governments, at least two legal scholars have suggested that the NPVIC would require explicit congressional approval because it would remove the possibility of contingent elections for President being conducted by the U.S. House of Representatives under the 12th Amendment.
First off, I would tend to consider the potential need for a contingent election as a bug, not a feature. But that aside, the claim seems wrong. It is certainly possible in principle for a popular-vote election involving the whole country to result in a tie; it's just extremely unlikely. But perhaps the wording of the NPVIC doesn't stipulate that in the event of a tie, the participating states' electors will be allocated so as to generate an Electoral College tie as well, if possible — and what happens if it isn't possible?
It may be that the NPVIC has to be replaced by an informal agreement, where the states that support the Compact all simultaneously but "unilaterally" change their system for appointing electors to one that implements a national popular vote. The trigger for this would that enough states have passed motions of intent to support this change "at some point". That would seem like no more of a Compact than one state lowering its sales tax when it notices a nearby state doing the same.
[0] https://en.wikipedia.org/wiki/National_Popular_Vote_Intersta...