Who benefits from the social democratic march to the middle?

Social democratic parties have experienced considerable electoral decline recently, which has often been attributed to their rightward policy movement. This paper advances this literature by examining who benefits from this moderation strategy and who is abandoning the social democrats. It does so by analyzing aggregate-level election results and individual-level Comparative Study of Electoral Systems data, on a sample of 21 advanced democracies, over 327 elections, from 1965 to 2019. I find little support for the assertion that social democrats are defecting to one party. However, in agreement with the spatial theory of party competition, results reveal that the radical left increasingly and significantly benefit from social democratic economic rightward positions, which is magnified when combined with rightward sociocultural positions. This predominantly occurs because left-leaning voters migrate to the radical left. The findings provide notable ramifications for party strategy and contribute to explanations for the rise of challenger parties, at the expense of mainstream parties.