Defining and measuring party system nationalization

In this paper, we propose a new measure of party system nationalization based on a minimalist definition of the phenomenon. A perfectly nationalized party system is a party system with only national parties or, in other words, without sub-national parties. Instead of the homogeneity of parties’ vote shares or the number of parties throughout the country, our measure captures the aspirations of parties to be national whose proxy is the proportion of districts (weighted by seats) that a party runs a candidate. The measure is compared with existing indicators through a longitudinal analysis of 256 elections in 18 Western European countries from 1945 to 1998.