AT THE CONSTITUENCY LEVEL
OUR MAIN ARGUMENT in this book is that the relative authority of national and subnational governments in a country helps determine the success or failure of regional and minor parties and, therefore, the formation of national party systems; however, we also concur with prevailing research that says electoral rules shape party systems. The literature on the effects of electoral rules on politics in single-member, simpleplurality systems has devoted considerable attention to testing Duverger's Law—whether or not these rules lead to two-partism. We assert that, although electoral rules directly affect district-level elections, they do not necessarily influence the formation of national party systems, and also that party aggregation at the national level is a function of the relative powers of national or state (provincial) governments. If a country is highly centralized and if the electoral rules reduce the number of district-level candidates or parties to a low number (near 2), then the number of national parties should be low (near 2) and similar to the number of district-level parties. If, however, a state or provincial government has greater authority over large areas of policy, then, even if the electoral rules reduce the number of local competitors, the country's party system could remain highly fragmented and regionalized. Different parties can coexist in different districts and different regions of the country.
Although the effects of electoral rules and centralization have resulted in two-partism in the United States, it does not follow that all countries that are characterized by centralized decision making and single-member districts with plurality rule would have only two parties competing nationally. A fully national party system, where national-level party competition is similar to local-level party competition, could settle on three parties, or more, at different levels of vote aggregation. What is important for our argument is the degree of similarity across these levels. If three parties were to win votes consistently at the district level, that could challenge Duverger's Law, but would not necessarily be an issue for our overall argument. If, however, two-partism dominates at the district level in these systems, then we should expect a Duvergerian twopartism nationally when authority is centralized.