What are the effects of a mobilized party base on elections? I present a new behavioral measure of the enthusiasm gap in a set of American elections to identify how the turnout rate of the party faithful varies across different contexts. I find that the advantaged party can see its registrants turn out by four percentage points more than the disadvantaged party in some elections, and that this effect can be even larger in competitive House districts. I estimate the net benefit to party vote share of the mobilized base, which is around one percentage point statewide, and up to one and one half points in competitive House contests. These results suggest that the partisan characteristics of an election have consequences not just for vote choice, but for the composition of the electorate.

Those who turn out in American primary elections are a small and unrepresentative subset of the population. Why do citizens forgo participation in nominating contests yet vote in general elections? We argue that limited contact lowers participation in primary elections. We present results from a randomized field experiment with near 150,000 letters in California’s 2014 primary. Each letter went to one of the four million Californians who had participated in recent general elections but not in primaries. We find that a single letter increased turnout by 0.5 points from a base rate of 9.3 percent. This increase is more than twice the average effect calculated in a recent meta-analysis and represents a proportional increase of 5.4 percent. Our experiment shows that registrants who typically abstain from primaries—and who are thus often ignored by campaigns—can be effectively mobilized.