Many people erroneously think that the Constitution requires the present
system of plurality, winner-take-all voting, but except for votes of the
Electoral College and some votes in Congress, the method of voting is left to
legislation, and other methods could be adopted.
Instant runoff voting — If no candidate gets a majority, the votes of those who voted for him as their second choice would be added to his count, and so forth until one candidate emerges as acceptible by the largest majority. This could result in the election of a candidate that was everybody's second choice, such as a minor party candidate.
Range voting — Each voter rates his approval of each candiate on a scale, and the ratings are averaged, with the candidate having the highest average (perhaps above a certain threshhold to allow for rejection of all candidates) being deemed the winner.