Dark horse

A dark horse is a little-known candidate who seems to come from nowhere at the last minute to contest a major party nomination. James K. Polk was the dark horse who unexpectedly won the Democratic Party nomination in 1844, and won the election.

Michigan Governor George Romney was a dark horse in 1968, and did not get the nomination

Unsuccessful dark horses include William Jennings Bryan (winning the Democratic nomination in 1896 but losing the election), and Wendell Willkie (winning the Republican Party nomination in 1940 but losing the election).