The United States has the right to force electors to vote in the presidential election as instructed by the states, and not of their own accord, according to a unanimous decision of the US Supreme Court.
The US presidential election is held according to a two-stage system – first there is a popular vote in each individual state, and the winner goes to all the electors from that state. State electors then assemble in Washington and confirm the will of voters by formal vote.
In the past, there have been cases where individual electors voted against the rule that all votes from a given state go to the winner, but this never affected the voting results. The US Constitution does not explicitly prohibit electors from violating the will of voters, but some states have laws of the relevant content.