On December 9, Colorado filed this brief in Baca v Hickenlooper, the case over whether presidential electors can vote for any qualified presidential candidate, or whether they can be replaced if they vote for someone different than expected.
The only U.S. Supreme Court precedent on this subject is Ray v Blair, 1952. Most people probably don’t realize when they read about Ray v Blair that in 1952 and previous years, the Alabama Democratic Party chose its candidates for presidential elector in a primary. Nowadays, no party chooses candidates for presidential elector in a primary. In Ray v Blair, the U.S. Supreme Court upheld a Democratic Party rule that no one could get on a party’s primary ballot (as a candidate for presidential elector) unless that person signed a pledge promising to vote for the party’s eventual presidential nominee. Thus Ray v Blair was about the right of parties to control who could seek their nomination. It was not about the ability of presidential electors to vote freely in December. Thanks to Politico for the link.
UPDATE: here is the brief of the intervenor, the Colorado Republican Committee.