Did the civil rights movement impact the development of the American state? Despite extensive accounts of civil rights mobilization and narratives of state building, there has been surprisingly little research that explicitly examines the importance and consequence that civil rights activism has had for the process of state building in American political and constitutional development. Through a sweeping archival analysis of the NAACP's battle against lynching and mob violence from 1909 to 1923, this book examines how the NAACP raised public awareness, won over American presidents, and secured the support of Congress. In the NAACP's most far-reaching victory, the Supreme Court ruled that the constitutional rights of black defendants were violated by a white mob in the landmark criminal procedure decision Moore v. Dempsey. This book demonstrates the importance of citizen agency in the making of new constitutional law in a period unexplored by previous scholarship.