We document that postwar U.S. national elections show a strong pattern of “incumbency disadvantage”: If the presidency has been held by a party for some time, that party tends to lose seats in Congress. To generate this, we employ Alesina and Tabellini's (1990) model of partisan politics extended to have elections with prospective voting. We show that inertia in policies (combined with su cient uncertainty in election outcomes) implies incumbency disadvantage. We find that inertia can cause parties to target policies that are more extreme than the policies they would support in the absence of inertia and that such extremism can be welfare reducing.