Candidate Competition and Attrition in Presidential Primaries, 1912-2000

This article analyzes candidate competition and attrition in presidential primaries from 1912 to 2000. We use a modified market concentration index to measure the number of effective candidates entering presidential primaries and to calculate winnowing across presidential primaries. We test competing hypotheses about the increasing number of candidates entering primaries. We find that the number of effective candidates increased following the reforms of the 1970s. We reject the hypothesis that the number of candidates entering primaries increased gradually as a function of long-term factors. We also find similarities in the winnowing of candidates in each party during the prereform and postreform eras, which suggests the timing of candidate withdrawal owes to more than differences in each party’s delegate allocation rules. Republican presidential primaries more efficiently winnow candidates than do Democratic presidential primaries in both the prereform and postreform eras.