Cross-cutting cleavages have long moderated party conflict in American politics and thereby contributed to democratic stability. Over recent decades, however, many of these cross-cutting cleavages have declined, sorting Americans into mutually antagonistic camps. Here I present data on the social composition of the parties in Congress indicating that they are increasingly differentiated by race, gender, and religion. Drawing on comparative work on democracy-building in divided societies, I argue that the deeper social divide between the parties makes American democracy more dependent upon political institutions that routinely impose power-sharing requirements on the major parties. Although polarization stresses the political system in numerous ways, American institutions still tend to block one party from running roughshod over the other. Such a political system risks immobilism and ineffectiveness, but it also promotes democratic stability.