Why have group-party alliances become more common since the mid-twentieth century? To address this question, 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 and security gains, which are especially valuable during periods of growth. I demonstrate the plausibility of this organizational theory by examining the evolution of group-party relations in the executive branch from the Nixon through Reagan administrations using archival documents collected at four presidential libraries.