Among the questions that vex the federalism literature are why states check the federal government and whether Americans identify with the states as well as the nation. This Article argues that partisanship supplies the core of an answer to both questions. Competition between today's ideologically coherent, polarized parties leads state actors to make demands for autonomy, to enact laws rejected by the federal government, and to fight federal programs from within. States thus check the federal government by channeling partisan conflict through federalism's institutional framework. Partisanship also recasts the longstanding debate about whether Americans identify with the states. Democratic and Republican, not state and national, are today's political identities, but the state and federal governments are sites of partisan affiliation. As these governments advance distinct partisan positions, individuals identify with them in shifting, variable ways; Americans are particularly likely to identify with states when they are controlled by the party out of power in Washington. States also serve as laboratories of national partisan politics by facilitating competition within each political party. In so doing, they participate in national political contests without forfeiting the particularity and pluralism we associate with the local. By instantiating different partisan positions, moreover, states generate a federalist variant of surrogate representation: individuals across the country may affiliate with states they do not inhabit based on their partisan commitments. Attending to the intersection of partisanship and federalism has implications for a number of doctrinal controversies, such as campaign finance across state lines and access to state public records. The analysis here suggests that porous state borders may enhance states' ability to challenge the federal government and to serve as sites of political identification.