The dominance of mainstream parties in Western European party systems is increasingly under threat due to the rise of challenger parties. With this dissertation, I offer a new perspective on how mainstream parties compete with challenger parties. My point of departure is that mainstream parties compete against challenger parties by portraying them as either incompatible with or even threatening to the democratic polity (a delegitimizing strategy) or as “normal” democratic competitors (a legitimizing strategy). I argue that these strategies have important consequences for how citizens perceive and evaluate challenger parties. Focusing on Germany and Sweden, I first map how the Swedish mainstream right Moderate party shifted from a delegitimizing to a legitimatizing strategy towards the far right Sweden Democrats. In contrast, the German mainstream right Christian Democrats maintained and even slightly intensified their delegitimizing strategy towards the far right Alternative for Germany over time. This raises the question of how citizens react to such mainstream party behavior. Two large-scale survey experiments, one implemented in each country, show that citizens indeed follow mainstream parties’ (de )legitimizing strategies in their own evaluations of challenger parties’ legitimacy. This finding holds across different political settings and different segments of the electorate. Overall, my findings thus suggest that mainstream parties have considerable leeway in shaping the extent to which citizens deem challenger parties legitimate. This dissertation seeks to advance our understanding of party competition in times of challenger parties’ disruption of party systems as well as public opinion formation.
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