This article presents an analysis of the professional and political activities of the demographer Roberto Bachi prior to Israel's establishment as a state in 1948. The article describes his involvement in two interconnected major areas: first, his advocacy of pro-natal policies, connected to a nation-building strategy by the Jewish population to achieve numerical dominance over Arab Palestinians in areas to be incorporated in the Jewish state, and second, the development of Jewish ethnic distinctions, particularly the 'Mizrahi type', to track differences in birthrates and changing cultural features within the Jewish population. The article also revises the historical record by showing the importance of this ethnic classification in the years prior to the large waves of Jewish immigration from Arab countries. Without the reworking of the popular category 'Mizrahi' into a scientifically systematized category by a demographer who would become the head of the state's Central Bureau of Statistics upon its founding in 1948, this binary social epistemology could not be as strong and legitimate as it actually was. Two factors account for Bachi's success. First was his ability to provide a new way of understanding the present in terms of the future. His numerical predictions on the Jewish and Arab demographic development made statistics and demography an indispensable technology for public policy and social planning. Second was his role as a boundary actor--a unique mediating position between political and scientific spheres. The Israeli case study exemplifies similar dynamics found in other countries during periods of structuring the modern state, namely, processes in which experts of infrastructural knowledge such as statistics and demography saw themselves as responsible for the national progress and its social modernity.