This essay maps the ways that antebellum politics and parties operated as federal systems and reveals the significance of this for understanding the rise of the Republican Party. Drawing on evidence from the late 1850s, it offers a guide to the ideas, structures, and mechanisms that made American politics operate as a federal system. Using federalism as a frame forces us to rethink common assumptions about the Third Party System and to see northern politics as competitive, turbulent, and regionally distinct. Acknowledging how federalism served as the scaffolding of political competition changes our view of the political parties. Adapted to the devolved and dispersed nature of American elections, parties functioned federally, composed of national, state, and local iterations that communicated and collaborated but acted largely without interference. The success of the Republican Party becomes a story not just of the triumph of antislavery principles but of the party's flexibility and skill in morphing its identity and messaging to suit local conditions.