When candidates in primary elections are ideologically differentiated (e.g., conservatives and moderates in the Republican party), then candidates with similar positions affect each others’ vote shares more strongly than candidates with different ideological positions. We measure this effect in U.S. Presidential primaries and show that it is of first order importance. We also show that voter
beliefs about the candidates harden over the course of the primary, as manifested in the variability of candidate vote shares. We discuss models of sequential voting that cannot yield this pattern of results, and propose an explanation based on a model with horizontally and vertically differentiated
candidates and incompletely informed voters. Consistent with the predictions of this model, we also show that, in more conservative states, low quality conservative candidates do better relative to high quality conservatives, and vice versa.