How do unions contribute to the capacity of left-wing parties to build new voters’ coalitions? It is conventional wisdom to consider unions as a burden on the capacity of left parties to forge new voters’ coalitions in de-industrialized times. The article proposes to reconsider the role of trade unions. Revisiting the study by Przeworksi and Sprague (1986), it is argued that strong unionization helps Left parties to win support simultaneously among the working class and the middle class. The argument is tested using data from the Comparative study of electoral systems (CSES) modules 2-4 in advanced industrialized countries from the early 2000s to the mid-2010s. The findings show that 1) there is a trade-off for working-class parties; 2) the trade-off is not mitigated by union membership; 3) but union membership impacts on the new voting patterns of disillusioned workers.