This dissertation argues that changing ideas about race and engagement with race science were at the heart of a major transformation of political science in the 1920s, a transformation that I characterize as "becoming modern." This transformation was at once conceptual--visible in the basic categories and theoretical apparatus of the discipline--and institutional--affecting the daily practices and institutional setting of political science. For the Gilded Age political scientists who built the first Ph.D. programs in the United States, historical development was racial development; political destiny was racial destiny. By the 1930s, however, "the political" had come to appear largely autonomous. It was no longer a function of nature or unfolding historical essence, but rather a human creation and therefore subject to rational management. I show that political scientists effected this transformation in large part by engaging a parallel and roughly contemporaneous transformation in racial thought. Specifically, it was by thinking through the Boasian critique of evolutionary anthropology that political scientists produced a "modern" conception of politics, delinked from notions of racial development. However, this is not a straightforward story of progress in which shedding prejudice leads to scientific advance. I show that these same interwar political scientists were deeply attentive to developments in "mental measurement" and eugenics. Of greatest interest to them were attempts to specify the capacities and limits of racial and other groups within the population, such as the World War I Army intelligence-testing program and successive attempts to create psychological and physiological tests that could measure capacities or predict responses. Animated by the possibility that citizens' capacities could be quantified and that this knowledge could be used to reform politics, influential political scientists worked to forge intellectual and institutional links with race science, including extreme figures within the eugenics and immigration restriction movements. This was particularly true in the early moments of institutional establishment of the discipline within a larger infrastructure for social science, as with the founding of the Social Science Research Council. As a result, I argue, this cohort did not so much abandon "race" as open space within modern political science for ascriptive hierarchy re-described in liberal terms. What emerged was a vision of the political as an independent realm conditioned by the "facts" of citizens' natural capacities. In my view, this vision has been both productive and limiting for the discipline, suggesting research programs that we still pursue but at the same time closing off other areas of inquiry. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]