Foundational studies in political science endeavored to explain the dynamics of voter turnout in America over time. Two theories, one focused on legal-institutional factors and the other on behavioral elements such as party mobilization strategies, were born from the need to account for temporal fluctuations in aggregate voter turnout. A comprehensive test of these competing theories has been hindered by the fact that reliable measures of the behavioral factors driving turnout have proven elusive. In this article, I develop and test an interactive theory of voter turnout that focuses on the impact of legal-institutional barriers to the franchise conditional on party organizational strength and mobilization efforts. To this end, I use data on the circulation of party-sponsored newspapers, coupled with information on the spread of voter registration requirements, to capture the effects of both behavioral and legal-institutional factors in Pennsylvania between 1876 and 1948. My results offer modest empirical support for an interactive theory of aggregate voter turnout. In isolation, however, the effects of behavioral factors are quite limited. On the contrary, legal-institutional variables exert a sizable impact on voter turnout in the state. Contrary to other recent work on the subject, careful analysis of the Pennsylvania case therefore provides a great deal of evidence in favor of legal-institutional accounts of the changes in aggregate voter turnout that were witnessed at the beginning of the twentieth century while demonstrating that behavioral factors, such as the decline of the partisan press, served to enhance the deleterious effects of legal reforms on turnout.