In recent years, scholars across the humanities have argued that the nineteenth-century American abolitionists articulated important conceptual lessons about democracy. This essay contributes to this literature by newly interpreting the political thought of Charles Sumner. Regnant scholarly treatments of Sumner have been narrowly biographical. I shift focus by examining his use of the word “caste” as an analytic and political term. The article demonstrates that Sumner adopted the language of caste from missionary accounts of caste hierarchy in India; that he used this information to argue that there was an oppressive analogue at home: racial caste; and that, accordingly, Sumner's conception of abolition included the dismantling of racial caste and the cultivation of interracial republican association.