Candidate competition and voter learning in the 2000–2012 US presidential primaries

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 US presidential primaries and show that it is of first-order importance. We also show that voters’ 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 that 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.