For West Germany, Windolf/Haas (1989), Lorenz/Wagner (1991) and Fitzenberger et al. (1999) report that Social-Democrat (SPD) voters have a higher probability of being union members, which is not surprising given the historically close relationship between the SPD and the labour movement.
exploredata-ZA3700_VarV519 exploredata-ZA3700_VarV42 exploredata-ZA3700_VarV106 exploredata-ZA3700_VarV517