現代先進国の新興政党：EU内外の政党の位置と有権者の認識

Over the last fifty years, the mainstream party families that have traditionally dominated politics in the West have experienced significant electoral setbacks.Social and Christian democrats, liberals, and conservatives have lost ground in most Western democracies.As mainstream parties decline, a variety of challenger parties spanning both ends of the political spectrum are on the rise across many advanced democracies.When looking at the electoral performance of challenger parties, Western Europe appears to be particularly affected by this phenomenon.Among established liberal democracies within the European Union (EU), the last decade has seen challenger parties more than double their vote shares compared to the 1970s.In contrast, challenger parties in equally long-standing liberal democracies outside the EU have increased their electoral returns by less than one-fourth over the same period.The thesis aims to explain why Western European countries are experiencing an unprecedented surge of challenger parties, while other leading liberal democracies including Japan, Canada, Australia, New Zealand, and Israel have not witnessed such phenomenon.The thesis provides the following answers: the observed disparities in challenger party support across EU and non-EU democracies depend, essentially, on fundamental differences in the behavior of mainstream parties in the two contexts.Mainstream parties in the EU have a strong tendency to converge ideologically amid the process of European integration.Mainstream party convergence leaves voters unable to distinguish among traditional center-left and centerright options and, as a result, a growing number of them look for alternatives outside the political mainstream.On the contrary, this tendency towards ideological convergence does not apply to mainstream parties in non-EU democracies and therefore voters in these countries are less likely to turn to challenger parties.Chapter 2 reviews four main approaches in the literature on challenger party support: electoral institutions, socio-economic factors, value cleavages and spatial theories of party competition.It suggests that while challenger parties are likely penalized by restrictive electoral rules, they have nevertheless prospered in highly disproportional electoral environments.However, most Western European countries adopt proportional representation and therefore one must consider the potential effects of electoral systems when comparing the electoral strength of challenger parties across EU and non-EU democracies.As for the socio-economic factors and the specific cleavages that may facilitate the emergence and success of different types of parties, I suggest that the socio-economic correlates of challenger party success examined in the literature, including economic crises and rising immigration levels, are commonly found across most advanced democracies.Thus, these perspectives offer little explanation for the observed disparity in challenger party support between EU and non-EU countries.On the other hand, spatial theories of party competition (Downs, 1957; Kitschelt, 1995) have the advantage of offering a unified framework for understanding the challenger party phenomenon: mainstream party convergence opens a space on the ideological axis, providing challenger parties with the opportunity to step in and gain support.

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