The 538 people who cast the actual votes for president as part of the Electoral College must vote as the laws of their states direct.
The "faithless elector" case was a defeat for changing the Electoral College. A win would force a shift towards a nationwide popular vote. But it was a win for state election officials who feared that empowering rogue electors would cause chaos.
Justice Elena Kagan said the Constitution gives states far-reaching authority over choosing presidential electors including the power to set conditions on an elector's appointment, "that is to say, what the elector must do for the appointment to take effect." she wrote, "nothing in the Constitution expressly prohibits states from taking away president electors' voting discretion." The ruling aligns with "the trust of a nation that here, We the People rule."
The general election is not actually a direct vote for the president, as voters choose a slate of electors appointed in their states by the political parties. The electors meet and cast their ballots, which are counted during a joint session of Congress in January.
The court's opinion said electors must act as their states require. In most of the nation that means voting for the candidate who won the popular vote in their states. In Maine and Nebraska, presidential electors are guided by the votes of congressional districts.
If the court ruled the other way, then individual electors who decided to vote as they wished in a close race could potentially have the power decide who wins.
Four "faithless electors" from Colorado and Washington state who did not conform to the popular vote in the 2016 election sued, claiming that states can regulate only how electors are chosen, not what comes later.
More than a dozen states have signed an interstate agreement to make the change. It would take effect once the participating states represent at least 270 electoral votes, the minimum needed to be elected president.