Emerging from a critique of the concept of “populism” in contemporary democratic theory, this dissertation develops a theory of radical democracy based on the politics of cooperative movements. I argue that cooperative politics help to clarify a central concern for democratic theory that is often obscured by theories of populism: how the popular sovereign can develop a form of social interdependence that facilitates their free and equal participation in self-government. Working through a political history of cooperative movements in the United States from the Civil War to the Cold War, my study traces a series of reformulations of the ideal of “the cooperative commonwealth,” beginning with the Populist movement and extending through the Socialist Party and the work of John Dewey and W.E.B. Du Bois. In the end, I argue that cooperative politics requires combining the local work of voluntary cooperation in multiple sites of self-organization with efforts to forge alliances among the popular classes centered on a non-exploitative vision of cooperative interdependence. Such a vision must clarify that cooperation is not simply shared instrumental activity, but a form of free association in which participants’ personal autonomy is secured by sharing in the inherently collective work of social reproduction. Cooperative movements’ reveal how the difficulty of achieving such cooperation is not a natural feature of human sociality, but an organizational defect of capitalist societies. Today, the project of cooperative democracy does not require an abstract blueprint for the cooperative commonwealth, but an archive of lessons based on the history of cooperative struggles.