This paper investigates variations in party membership levels across 120 states/provinces in six countries: Belgium, Germany, Spain, Brazil, Chile, and Mexico. By relying on official data at the subnational level, we explore the impact of institutional, demographic, and socioeconomic variables on the aggregate levels of party membership (ratio members/electorate in each polity). As membership figures are strongly country specific, each variable expresses the distance (difference) between the value of the specific state/province and the value of the corresponding country as a whole. The first findings show that urbanization and number of municipalities tend to increase party membership levels, while size and density of population have negative effects. The most urbanized states have more party members than the other states in a given country; however, membership ratios decrease in the extremely dense polities, where the metropolitan areas are located. Despite the great variation between Latin American (more members) and European (less members) democracies, the main results of the multivariate models can ‘travel’ across the two regions. The findings suggest that ecological studies, by highlighting contextual factors, may contribute to explain within-country variations in party membership, when many institutional variables remain constant.
