This paper provides a dynamic theory of a parliamentary government system with proportional-representation elections, policy-motivated parties and voters, and an endogenous status-quo policy. The theory identifies the representation of parties in parliament, the governments the parties form, the policies chosen by those governments, and the duration of the governments and their policies. Governments are majoritarian, government parties are equal partners, they and their policies are durable, voters elect minority parliaments in every period, and government policies provide concessions to centrist voters. If crises can occur, governments can fall, but a new government forms after the next election. The theory provides explanations for three empirical findings: equilibria consistent with Gamson’s law, an analog of Duverger’s law for proportional-representation electoral systems, and compensational voting where voters give the out party additional votes when an incumbent government is expected to continue in office.