When the welfare state is under attack from neoliberal reformers, how can trade unionists and other campaigners build solidarity to defend it? Based on 45 qualitative interviews, this article compares campaigns to defend British health services and social security benefits between 2007 and 2016. Building on the macro-insights of comparative welfare-state literature and the more micro-level insights of studies on mobilisation, community unionism and union strategy, it examines the factors that help or hinder the construction of solidarity. This research finds that building solidarity is more difficult when defending targeted benefits than universal ones, not only because of differences in public opinion and political support for services, but also because the labour process associated with targeting benefits, namely the assessing and sanctioning of clients, can generate conflicts among campaigners.