Yes, it is probably constitutional. Each state can decide how its electoral votes are allocated. If Maine and Nebraska can allocate electoral votes by congressional district, without any problems, then states should be able to allocate electoral votes based on popular vote, without incident.
I suspect there are limits on how a state might allocate its EVs. Could a state allocate its EVs based on the value of the Dow Jones average on election day? I doubt it. Sticking to elections, could a state allocate its EV to the results in its most populous county? That clearly violates one man one vote. Could a state allocate it based on another arbitrary state? That seems like a violation, too, since it is equivalent to letting that other state have its votes count twice.
So, let me assume that the states have a recognized compact. That could potentially allow the states to aggregate their votes and electors to act as a single state. However, if they use results from states not in the compact, isn't that akin to using votes from some arbitrary state to make their allocation? If so, that also becomes a potential violation of one man one vote, since again the states not in the compact effectively elect electors twice.
To me the only clearly constitutional compact is one that allocates the electors from the compact to winner of the vote within the compact. However, even if the compact has 270 EV, the result could be that the compact winner is not the national popular vote winner. That defeats the whole purpose of the NPVIC.
That's impossible. If the compact reaches 270 EV's, the states within the compact will not vote according to the popular vote of the states within the compact, but will instead vote according to the popular vote of the ENTIRE COUNTRY (including states not within the compact).
