Electors under our Constitution are actually free to vote for whomever they wish, even themselves if they want to.  It's a poorly understood fact about our system.  They're called "faithless electors" when they do this.  Wikipedia has a history of all who have done so.  They have never changed the results of an election, but that doesn't mean they couldn't if they so chose.

https://en.wikipedia.org/wiki/Faithless_elector