I am not so sure this would violate the Constitution. States determine how and who is an elector and they then can make them vote in a certain way by law if they so choose, or they can let them vote however they want.
Today, an elector in DC for an example can vote for whoever the elector wants, or no one if that is what the elector wants.
In fact, that happened in 2000 when the DC Elector voted for no one instead of Algore who won the vote in DC.