Electors don't vote on party lines, the state legislature for each state tells the electors how to vote.

If they attempt to vote any other way, the state lege can step in and tell the Electoral College to dismiss that vote and elector

The U.S. Supreme Court has unanimously upheld laws across the country that remove or punish rogue Electoral College delegates who refuse to cast their votes for the presidential candidate they were pledged to support.

The decision Monday was a loss for "faithless electors," who argued that under the Constitution they have discretion to decide which candidate to support.

Writing for the court, Justice Elena Kagan said Electoral College delegates have "no ground for reversing"

Kagan's opinion noted that the original Electoral College system created by the framers of the Constitution failed to anticipate the growth of political parties. By 1796, the first contested election after George Washington's retirement, the system exploded in disarray, with two consecutive Electoral College "fiascos."

That led to passage of the 12th Amendment in 1804, "facilitating the Electoral College ... as a mechanism not for deliberation but for party line voting," Kagan wrote.

Nothing in the Constitution prevents the states from "taking away presidential electors' voting discretion..."