This paper investigates the position of social democratic parties (SDPs) towards antitrust (competition) policy. Given their traditional state-interventionist attitude and their ties with organized labour, SDPs have long been considered as not supportive of antitrust policy. However, antitrust policy’s goal of granting consumers lower prices is beneficial to salary earners. Hence, it is not surprising that SDPs’ support for antitrust policy varies considerably. To account for such variation, this paper hypothesizes that SDPs’ support for antitrust policy depends on: (a) the influence of trade unions; (b) the electoral system; and (c) the degree of coordination of the economy. Analysing in depth 16 party manifestos of West European SDPs from 2002 to 2013, we check the plausibility of our hypotheses with seven paired comparisons. Our analysis supports the hypothesis that the influence of trade unions affects SDPs’ support for antitrust policy, while the impact of electoral system and economic coordination appears less evident.