Recent elections have revived the debate about the decline of social democracy, often attributed to the numerical decline in the working class and its alienation due to the mainstream left’s economically centrist and socially liberal policy stances. To explain changes in these parties’ fortunes, we instead argue that researchers need to analyze the preferences of key electoral groups on the main axes of political competition and the role of information-transmitting intermediaries in shaping these preferences. Specifically, we suggest that (1) mainstream left parties can win votes by taking up more investment-oriented positions if they (2) also take up liberal sociocultural positions and (3) do not face opposition from influential unions. We find support for these expectations using aggregate-level election results and individual-level survey responses. Our findings have important implications for our understanding of party success in advanced democracies and for empirical models of party competition more generally.