The U.S. Supreme Court has held that the Constitution does not require that Electors be completely free to act as they choose and therefore, political parties may extract pledges from electors to vote for the parties' nominees. Some state laws provide that so-called "faithless Electors" may be subject to fines or may be disqualified for casting an invalid vote and be replaced by a substitute elector. The Supreme Court has not specifically ruled on the question of whether pledges and penalties for failure to vote as pledged may be enforced under the Constitution. No Elector has ever been prosecuted for failing to vote as pledged. 
Today, it is rare for Electors to disregard the popular vote by casting their electoral vote for someone other than their party's candidate. Electors generally hold a leadership position in their party or were chosen to recognize years of loyal service to the party. Throughout our history as a nation, more than 99 percent of Electors have voted as pledged.