2020 UPDATE AND CORRECTION: There is no law that affects the UNIMPAIRED power of a presidential elector to vote for whichever presidential candidate they choose in accordance with their individual judgment.  That is Constitutional law (Article II) that has no other law above it.  This issue is before the Supreme Court, on a petition for CERTIORARI, Chiafalo vs Washington (State) #19-465, where the Court is asked to decide whether the electors are free agents when they cast their ballots for President and Vice President, or is their ballot choice controlled by the state that appointed them. Briefs and motions have been filed, and oral arguments begin May 13, 2020. Documents filed in this case can be found available to the public at the SCOTUS website.

 

There is no law that affects the UNIMPAIRED power of the state legislature to cast their presidential electoral votes for whichever person they choose.  That is Constitutional law (Article II) that has no other law above it. 

 

The Two-Party System is a legal fiction.  There is no state or federal law that tells the people how to organize political factions, groups or parties.  There could be no such state or federal law because such a law would be opposed to the First Amendment and Article II of the Constitution. 

 

There are state laws that require a political party to meet certain organizational standards which any political faction or group must meet in order to have a candidate's name or the party name printed on an official ballot.  There are no such laws and there cannot be any such laws for write-in candidates.

 

About 25 states have laws that say the presidential electors must cast their electoral votes for the candidate that won either the majority or the plurality of the votes cast by the people.  But all of those laws apply only to the major political parties and parties registered with the state.  The only law that tells the state legislature how to cast their electoral votes for a write-in candidate who wins the popular vote is Article II of the Constitution, which states that the legislature may cast their electoral votes with no restrictions as "they may direct."  There is no law that affects the UNIMPAIRED power of the state legislature to cast their presidential electoral votes for whichever person they choose.  That is Constitutional law that has no other law above it.  The people are free to form political parties and party coalitions however they wish.  The First, Ninth and Tenth Amendments protect those rights, which were not delegated to any government body.  Further, a state legislature of past years IS NOT the current state legislature, and the specific political authority to cast the presidential electoral votes AS THEY MAY DIRECT belongs to the current legislature, and not a state legislative body of the past.