"Focusing on Germany and the United Kingdom as two most dissimilar cases in terms of labour market and political institutions, the article examines the impact of trade union membership on partisan preferences. Leveraging panel data to control for time-invariant selection effects shows that trade unions exert a small but consistent left-wing influence on wage earners who become affiliated, but they are no longer able to modify the preferences of working-class members. A longitudinal approach reveals that unions mainly attract individuals who already share the unions' political inclinations before joining. The additional shift to the left experienced by already left-leaning new members is consistent with a value congruence mechanism triggered by interactions with even more left-leaning long-term union members. Symmetrically, working-class joiners exhibit less pronounced left-wing inclinations before becoming affiliated, a gap that widens further after they join. These findings imply that unions' political influence and class allegiances have been both eroded and altered by changes in the labour market and political landscape."

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