The SCOTUS would shoot this down faster than I can down a dozen wings.
The Constitution simply says:
Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector.
Now, this can backfire on them bigly. What if TRUMP wins the POPULAR VOTE? Then ALL their Electoral Votes go to HIM regardless of what their citizens votes were....................
The intent of the words "each state" was not to allow allow them to enter into multi-state compacts to repeal or replace the traditional constitutional electoral process without a constitutional amendment.
Although I believe the no interstate compacts clause dominates the "shall select electors however they choose" wording, I have to ask: Would that be true of a packed, politically apportioned 15 man SCOTUS? I'm not so sure.