Social democratic parties have changed their electoral appeals substantially to cater to new voter segments. This article examines social democracy’s fortunes in attracting new voters among the salaried middle class across different electoral systems. Previous research ignored the importance of electoral systems and was inconclusive as to whether social democratic parties succeeded in mobilizing new constituencies. I argue that electoral systems play a crucial role since proportional systems enhance the electoral competiveness of left-libertarian parties, social democracy’s most serious challengers among the salaried middle class. In contrast, majoritarian systems allow social democratic parties to gain a foothold among these voters as left-libertarians remain marginalized. Using ISSP data for 11 Western democracies, the findings demonstrate that social democratic parties were outperformed by their left-libertarian challengers among the salaried middle class under highly proportional systems, but not under majoritarian systems.