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.

[0] https://en.wikipedia.org/wiki/National_Popular_Vote_Intersta...