An innovative examination of the shift by American political parties toward issue-based differentiationRecent Democratic and Republican party platforms display clear differences on such issues as abortion, LGBTQ+ rights, gun control, and the environment. These distinctions reflect a programmatic party system—that is, one in which policy positions serve as a key basis of electoral competition. Yet party politics were not always so issue-oriented; the rise of policy positions as the dominant marker of party appeal occurred largely over the last fifty years. In Divergent Democracy, Katherine Krimmel examines this transformation of the American party system, using innovative machine learning techniques to develop and present the first measure of party differentiation on issues since Democrats and Republicans began competing with each other in 1856. Why did the shift to issue-based party competition take more than a century to materialize? Krimmel offers a groundbreaking theory, focusing on what aids and constrains parties’ abilities to do the difficult, conflict-ridden work of developing issue positions. She argues that clientelistic subnational party organizations, promising material support or jobs in return for votes, long impeded programmatic partisanship while the growth of national party organizations facilitated it. Moreover, institutions and agents of racial oppression extended the life of nonprogrammatic practices, as they attempted to shield discriminatory laws and institutions from interparty competition. Following the civil rights revolution of the 1960s, space opened for programmatic competition to grow. Using both quantitative and qualitative tools, Krimmel offers a vital view of the foundations of today’s issue-based party competition and its alternatives.