The political power of labor unions is a contentious issue in the social sciences. Departing from the dominant focus on membership size, we argue that unions' influence on national lawmaking is based to an important degree on their local organization. We delineate the novel hypothesis that the horizontal concentration of union members within electoral districts matters. To test it, we draw on administrative records and map the membership size and concentration of local unions to districts of the U.S. House of Representatives, 2003Representatives, -2012. We find that, controlling for membership size, representatives from districts with less concentrated unions have more liberal voting records than their peers. This concentration effect survives numerous district controls and relaxing OLS assumptions. While surprising for several theoretical perspectives, it is consistent with theories based on social incentives. These results have implications for our broader understanding of political representation and the role of groups in democratic politics. * We are grateful to Duke University and the University of Konstanz (YSF) for financial support. MB also acknowledges financial support from the Agence Nationale de la Recherche (ANR)-Labex IAST. We thank our editor and five reviewers, as well as Nicole

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