This article discusses a moment of constitutional development in which popular constitutionalism, that is, constitutional interpretation outside of the Courts, had a profound impact on the development of constitutional law. It describes a group of antislavery activists who argued that slavery was unconstitutional prior to its abolition by the Thirteenth Amendment. Though the Supreme Court rejected their arguments in the Dred Scott decision, the antislavery constitutionalists strongly influenced the Reconstruction Era Congress. James Ashley, a member of the Reconstruction Congress, spearheaded the successful effort to transform antislavery constitutionalism into law by enacting the Thirteenth Amendment. The article examines the tenets of antislavery constitutionalism and details how Ashley articulated that philosophy in speeches which he gave on the campaign trail and during congressional debates over that Amendment. This history not only illustrates the salutary impact that popular constitutionalism has on constitutional development, but it also provides lessons for understanding the meaning of the Thirteenth Amendment. First, the Amendment not only ended slavery, but it was also a transformative amendment which established freedom. Second, the Amendment bestowed broad power on Congress to define that freedom, incorporating into the constitution an institutional role for the popular constitutionalism that contributed to its formation.