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.