This dissertation addresses the electoral consequences of social democratic parties' shifts to the political center in left/right positioning. Although the move to the center of many European social democratic (SD) parties in the 1990s was first rewarded with victories, these parties have since faced a remarkable electoral drought. The project analyzes the votes of individuals in domestic and European Parliament elections from the first 15 members of the European Union (EU-15) countries over the past two decades in comparison to an individual's distance from social democratic parties' left/right position. Combining a number of insights from recent literature on voting and party competition with a modified version of Kirchheimer's "catch-all" thesis informed by historical institutionalist scholarship, I argue that the policy platform choices of social democratic leadership were decisive in explaining electoral outcomes and that a rightward shift of the Left's mainstream political parties in the mid-1990s contributed substantially to social democracy's slide over the last ten years. The gains these parties derived from the policy shift toward the middle in the 1990s were short-lived and came at the expense of electoral success in the subsequent decade, blurring the ideological coherence of the parties as political organizations in the process