dark horse, in U.S. politics, a person unexpectedly chosen by a major party as a candidate for public office, especially for the presidency. A presidential dark horse is usually chosen at a party national convention and often has acquired only a local or limited reputation at the time of his nomination. He is invariably the offspring of compromise after rival factions have deadlocked the convention. Probably the best-known example of a dark horse is James K. Polk, who was selected at the Democratic convention of 1844 on the ninth ballot, although he had not been nominated until the eighth ballot.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.