This article investigates to what extent social democratic parties still benefit from the support of union members at the polls. Not only are social democratic parties confronted with new competitors in the party systems, but also the union confederations of the socialist labour movement are in some countries losing their dominant position due to the rise of separate professional confederations. It is argued in the article that the effect of union membership on voting choice is conditioned by the structure of the trade union movement. The support of union members for social democracy is fostered by the strength of the confederations historically close to this party family, while it is hampered when strong separate (or politically unaffiliated) white-collar confederations exist. Using European Social Survey and Swedish Public Opinion data, the article shows that social democratic parties still enjoy important support from trade union members, but at the same time are under fierce competition from bourgeois and green parties among members of white-collar confederations. This reinforces the challenges for social democracy to build new voters’ coalitions in post-industrial societies.