A primary runoff is an election that occurs after a primary in which no candidate reached the necessary threshold for nomination defined by state law, which is typically more than 50 percent of the vote. The primary runoff usually involves the top two candidates from the initial primary vote.
States that require a runoff if no candidate receives more than half the vote include Alabama, Arkansas, Georgia, Mississippi, Oklahoma, South Carolina and Texas. Vermont requires a primary runoff only if two or more candidates tie. In South Dakota, a runoff is triggered if no candidate receives more than 35 percent of the vote. Louisiana, which holds an open primary, holds its primary runoff six weeks after the primary vote.