This paper compares the territorial structure of European political parties. In addition to existing explanations of party nationalization that focus on country-level institutional variables, we argue that aspects of party organization such as the degree of centralization and the type of factionalism also matter. By bringing the analysis to the party level, the paper provides a multilevel analysis of institutional and party organization variables and disentangles the effect of each set of influences. The analysis uses original data on party organization from recent expert surveys and links it with original data on party nationalization for 152 parties across 21 European countries. This research contributes to the literature on party system nationalization, party development and party formation.
