What are political parties, and how and why do they change? These questions are foundational to party research, yet scholars of American parties disagree about the answers. In this paper we present a new theoretical framework capable of bridging these scholarly divides and coming to terms with American party politics today. We argue that political parties should be seen as fundamentally contentious institutions. Due to their mediating position between state and society, parties are subject to rival claims of authority from a range of political actors, including elected officeholders, party officials, interest groups, and social movements. To manage intraparty contention, win elections, and govern, entrepreneurs construct and maintain party orders -- institutional and ideational arrangements that foster an operational degree of cohesion and constraint through time. Together, the dynamics of intraparty contention and the rise and fall of distinct party orders over time illuminate the patterns of American party development.