Immigrant group size and political mobilization: Evidence from European migration to the United States

•This paper studies immigrant group political mobilization.•I propose naturalization status as a measure of political mobilization.•Immigrants are more likely to mobilize as their share of the electorate grows.•Coalitions with Democrats and social networks matter for mobilization.

Immigration to democratic nations generates new groups of potential voters. This paper investigates how the electorate share of immigrant groups influences their likelihood of becoming politically mobilized, focusing on the mechanism of coalition formation with the Democratic Party. Using newly assembled data on ethnic enclaves in American cities at the start of the twentieth century, I show immigrants were more likely to mobilize politically as their share of the local electorate grew larger. This effect is driven by political mobilization in voting districts where the Democratic Party likely needed an immigrant group's vote to win elections. I also consider the shape of the electorate share effect, showing it is nonlinear and consistent with a political economy model of coalition formation.