WASHINGTON -- The 538 people who cast the actual votes for president in December as part of the Electoral College are not free agents and must vote as the laws of their states direct, the Supreme Court ruled Monday.

The unanimous decision in the "faithless elector" case was a defeat for advocates of changing the Electoral College, who hoped a win would force a shift in the method of electing presidents toward a nationwide popular vote. But it was a win for state election officials who feared that empowering rogue electors would cause chaos.

Writing for the court, Justice Elena Kagan said the Constitution gives states far-reaching authority over choosing presidential electors. That includes 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."

What's more, 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," Kagan said.

The November general election is not actually a direct vote for the presidential candidates. Voters instead choose a slate of electors appointed in their states by the political parties. Those electors meet in December to cast their ballots, which are counted during a joint session of Congress in January.

The court's opinion said presidential electors must act as their states require, which in most of the nation 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 had 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.

Harvard Law professor Larry Lessig, who advocates Electoral College reform, told the court that nothing in the Constitution gives states any authority to restrict how an elector can vote, because they act in a federal role when meeting as the Electoral College.

Instead of voting for Hillary Clinton, who won the popular vote in Colorado, Micheal Baca cast his vote for John Kasich, the former Republican governor of Ohio. And in Washington state, where Clinton also won the popular vote, three of the state's 12 electors voted for Colin Powell, the former secretary of state.

Lessig said on Monday that he was pleaded with the timing of the court's decision, but not the result.

"When we launched these cases, we did it because, regardless of the outcome, it was critical to resolve this question before it created a constitutional crisis. We have achieved that," he said. "Obviously, we don't believe the court has interpreted the Constitution correctly. But we are happy that we have achieved our primary objective: This uncertainty has been removed. That is progress."

The Supreme Court ruled in 1952 that states do not violate the Constitution when they require electors to pledge that they will abide by the results of the popular vote. But the justices had never before said whether it is constitutional to enforce those pledges.

Lessig said he hoped the controversy would encourage more states to adopt a system in which they would assign all of their electors to the candidate who wins the nationwide popular vote for president.

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.

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