Cross-District Coordination and Party System Fragmentation -- Evidence from Polish Municipal Council Elections

The conventional wisdom in electoral studies links proportional representation to multi-party systems and plurality rules to bipartism. Although this prediction fits the dynamics of political competition well at the district level, the theoretical mechanisms proposed by the available theory are not directly relevant to the formation of party systems beyond the district level. In a similar vein to the recent literature on party system nationalization, this paper argues that electoral rules influence party systems indirectly by structuring cross-district coordination. This article argues that candidate-centered plurality rules tend to impede cross-district coordination by encouraging independents, whereas proportional representation can encourage such coordination by advantaging larger electoral coalitions. As a result, in the early stage of party system institutionalization, proportional representation can reduce the size of party systems, in contrast to plurality rules that fragment the party system. I test the argument in the setting of Polish municipal council elections. I use a regression discontinuity approach exploiting the fact that the law prescribed a population threshold below which the electoral formula was plurality rules and above which was proportional representation. Results show that councils elected by plurality rules are just as fragmented, if not more than, those elected by PR systems. Further evidence suggests PR systems empower partisans and discourages independents that thrive under plurality rules. These findings highlight the importance of understanding the complexity of the implications of electoral rules in un-institutionalized party systems.