This study examines the social origins of interest group politics in the USA. Between 1890 and 1925, a system centred on elections and party organizations was partially transformed by increasingly prominent legislative and administrative policy-making, and by the insistent participation of nonpartisan organizations, including farmers, workers and women, who invented strategies to circumvent the parties. Voters learned how to monitor legislative processes, to hold their representives accountable at the polls, and to institutionalize their ongoing participation in shaping policy. The text analyzes organizational politics in three American states, California, Washington and Wisconsin, seeking to demonstrate how the political opportunity structure of federalism allowed regional innovations to exert leverage on national political institutions.