Why have group-party alliances become more common since the mid-twentieth century? This dissertation employs both qualitative and statistical tools to address the puzzle of contemporary special interest partisanship. After tracing partisanship across several measures, I develop a continuum of group-party relationships, running from fluid, unstructured interactions (akin to political pluralism) to highly institutionalized alliances (as we might see in a firm). Drawing on pluralist scholarship and theories of firm formation and evolution, I explore the costs and benefits of different arrangements, and explain why we might expect to see movement along the continuum over time. On the one hand, pluralism offers flexibility to parties and groups, and alliances have little value when parties are too weak to discipline their members in Congress. On the other, institutionalized alliances offer significant efficiency gains, which are especially valuable during periods of growth. I argue that changes in group-party relations stem from the growth of national party organizations over the second half of the twentieth century, which increased the value of group resources and intensified parties' need for efficiency. Until this period, parties were weak on the national level and strong on the state and local levels, and patronage was the primary currency of politics, leaving little room for issues in political competition. The New Deal's historic expansion of federal power disrupted this balance, temporarily strengthening local parties by offering new sources of patronage, while also sparking gradual, interconnected processes that would ultimately undermine machine power--most notably, the growth of groups and the rise of issue politics as a site of electoral competition. Realizing the economies of scale necessary to build strong national parties required movement away from pluralism into more structured, long-term relationships. Moreover, in order for the new site of competition to help Republicans build a coalition to compete with the long-dominant New Deal Democrats, distinct issue positions were necessary. The result of this party-building process is a pattern of group-party alliances quite unlike the bipartisan relations V.O. Key, David Truman, and others observed in the mid-twentieth century