This essay considers how the cultural authority of the constitutional Founding became legal authority in antebellum America. Examining a series of cases implicating the constitutional politics of slavery, it illustrates how legal professionals grasped the public power of constitutional origin stories. To produce meanings and legitimate rulings, lawyers and judges wrote and reproduced narratives about slavery at the Founding, converting ascriptions of original constitutional visions in formal constitutional law. This power derived from the ongoing popular construction of the Founding as a venerated and authoritative moment containing unwritten intentions, understandings, and promises binding upon subsequent generations. The essay argues that these developments belong to the deep history of originalism. By approaching originalism as a form of constitutional politics integrating public memory culture and legal reasoning, the essay locates the central public and juridical dynamics of originalism emerging in struggles over the constitutional identity of slavery.