In the late nineteenth century, Americans opposed to universal suffrage launched a new line of attack, asserting that women lacked the rationality required for enfranchisement because of innate biological disabilities. After the Civil War, debates about women’s mental and physical capacity shifted from those rooted in divine ordinance and Enlightenment rationality to those grounded in science and evolutionary theory. New research conclusions combined with an existing system of legal dependency to form a powerful new discourse of exclusion which still echoes today. As the twentieth century turned, practitioners of the newly prestigious scientific disciplines provided rationales for anti-suffragists to label these sex-based differences as physical and mental disabilities. Women’s rights advocates rejected these characterizations, but rarely refuted the assumption that disability justified exclusion. This article approaches these debates with disability as the center of inquiry to explore the intertwined nature of legal and biological power.