Scholars have devoted considerable attention to the consequences of delegate selection rules for presidential nominations, yet few have sought an explanation for the variance in these rules across the states and over time. In this article, we ask why state party elites would open their processes of delegate selection to a large and potentially ideologically diverse constituency by holding primary elections rather than caucuses. We develop an account of endogenous institutional choice that suggests elites ought to be increasingly likely to open their delegate selection rules as the ideological nature of the party and the state's electorate converge. We test this claim using a new data set on Democratic Party selection rules between 1972 and 2000 and find that the degree of ideological convergence is a strong predictor of state party choices to open the process of delegate selection. These results provide additional support for general theoretical claims that characterize political institutions as fundamentally endogenous to the politics they regulate.