Focusing on the effects of union membership on partisan preferences, this article explores how changes in Swedish industrial relations and trade-union politics have affected electoral support for Left parties since the mid-1980s. Our analysis shows that unionization among blue-collar workers has declined sharply since the mid-1990 and that this development has contributed to the decline of electoral support for the Social Democrats and for the Swedish Left as a whole. In addition, we find that the association between union membership and voting for Left parties has declined among white-collar employees without tertiary education as well as bluecollar workers over the same period. We argue that sectoral blue-collar and whitecollar unions alike have responded to membership losses and intensified competition between unions by engaging in practices that render the partisan preferences of union members less distinctive than what they used to be (less Left-leaning relative to non-unionized counterparts).