The electors are pledged to vote for a particular candidate, but they can vote however they like. Generally, they are selected by the party from among party activists and can be relied to to vote as instructed.
The Constitution says that electors will be selected in a manner decided by the legislatures of the several states. They can let the governor appoint them or appoint them themselves.
I worry a little about Massachusetts, or any other state, giving “full faith and credit” to an election total certified by, say, the Republican Secretary of State of Florida. The mechanism for certifying the national popular vote isn’t at all clear to me, but it does seem to nationalize ballot box stuffing. Stuffing ballot boxes in rotten boroughs like Philadelphia or New York City only had a limited impacted. Now those ballots count against everyone’s legimate ballot, nationwide.
“The mechanism for certifying the national popular vote isn’t at all clear to me...”
Because it does not exist. If one did exist, it would exist in the COTUS.