Any state’s legislature could, theoretically, pass a law setting out a new method for designating presidential electors other than popular vote. However, they would have enact such a law prior to Election Day; they could not retroactively change, or just disregard, their current laws to defy the will of voters. State election laws and regulations must be established and in place prior to Election Day — they cannot be improvised or instituted on an ad hoc basis after the fact. Otherwise, every presidential election would descend into chaos, with state legislatures controlled by one party refusing to appoint electors pledged to the other party’s candidate, as the Lawfare blog observed:
The legal theory that would allow state legislatures to go rogue and appoint electors without regard for the popular vote rests on an argument made by Chief Justice William Rehnquist in Bush v. Gore, for himself and two other justices. On this view, a legislature is unconstrained in its power to set the manner by which electors are selected — meaning that even after an election, the legislature could ignore the results and select a different slate altogether. A recent opinion by Justice Brett Kavanaugh suggests that Rehnquist’s argument may be again on the rise.
But advocates for this view need to recognize that between Bush v. Gore and today, the Supreme Court has unanimously decided that presidential electors are not actually “electors” but are instead bound to the people’s vote. That principle that cabins elector discretion must also constrain legislatures — at least if the country is to avoid an abomination that the Framers expressly rejected.
That said, the law is only relevant to the extent that is it enforceable. If Republican-dominated legislatures were determined to find excuses for ignoring their states’ election results, a Republican-controlled Senate were willing to facilitate the scheme, and a conservative judiciary were compliant in upholding the results, then such a plot might indeed succeed
