Though retrospective performance evaluations are now widely appreciated as a major influence on voting in general elections, their influence in presidential primaries has rarely been noticed. Using exit polls conducted by major media organizations over the last nine election cycles, this article shows that retrospective voting is an important, indeed dominant, factor in two types of situations: when an incumbent president is running for reelection, and when an incumbent vice president is seeking to become his party's next presidential candidate. This finding, in turn, helps explain two significant institutional features of the contemporary presidential nomination process: why most recent presidents have been renominated without much difficulty, and why the vice presidency has become such a good launching pad for presidential candidacies.