Trade unions have been analysed quantitatively primarily in their role as vested interest organisations, attempting to quantify the excludable benefits they provide to members rather than examine their wider impact in an institutional context. Power resource theory acknowledges unions as social agents but assumes the willingness to oppose neoliberalism is constant, limited only by scarce power resources. Whilst true in general terms, this fails to explain trends of increasing labour market dualism in resource‐rich industrial relations regimes. This article examines social solidarity as a union power resource, measuring the impact of trade union membership on social attitudes of solidarity. Data were collected from the 2016 European Social Survey for 18 countries, grouped into five distinct industrial relations regimes. The findings suggest that, at European level, union membership still has a significant effect on all dimensions of social solidarity, but these relationships vary significantly across industrial relations regimes.