This chapter identifies why the descriptive changes observed in Chapter 2 took place. To answer this question, it proposes a model of structural change being responded to by the key actors during the nomination process. These structural changes are grouped into three categories: changing electoral incentives, new regulatory reforms, and technological developments. These changes elicited responses from actors within the party network in terms of organizational structure and electoral strategy, by candidates in terms of whether and how they ran for office, and among voters in terms of their participation and motivation in nationalized congressional primaries.