We propose a theory of political parties in which interest groups and activists are the key actors, and coalitions of groups develop common agendas and screen candidates for party nominations based on loyalty to their agendas. This theoretical stance contrasts with currently dominant theories, which view parties as controlled by election-minded politicians. The difference is normatively important because parties dominated by interest groups and activists are less responsive to voter preferences, even to the point of taking advantage of lapses in voter attention to politics. Our view is consistent with evidence from the formation of national parties in the 1790s, party position change on civil rights and abortion, patterns of polarization in Congress, policy design and nominations for state legislatures, Congress, and the presidency.