Preference Shocks that Destroy Party Systems

We propose a two party electoral competition model to analyze the effects of an exogenous shock over voters’ preferences on the strategic policy choices of the parties. We find that if the shock affects voters’ ideology regarding an issue that is already salient, then both parties strategically adapt their already moderated policy choices in the direction of the new median voter. However, if the shock changes the relative issue salience, then both parties strategically shift their policy choices from their ideal points towards the ideal point of the median voter of the newly salient issue. The asymmetry of the distribution of the voters preferences, that is possibly intensified by the shock, produces a disadvantage for one of the parties, which is forced to implement a large policy shift. We argue that a large policy shift may break a party internal balance among its different factions, which in turn may produce important disruptions in the party system. We illustrate our arguments with an analysis of recent events in Catalonia and the UK.