Competition between unions whose membership has different skills and professionalization levels is a long-standing issue in the labour movement. This article investigates the conditions for why and how a unique cross-professional coalition of all Danish public-sector unions developed between 2017 and 2018. Operating in a favourable context, unions overcame professionalization differences when skilled brokers primed a common instrumental base as other unionists used a public interest frame to legitimate the coalition and its demands ideologically. However, once the common instrumental concern was met, the coalition collapsed. The article argues that union coalition-building depends on multiple factors comprising both contextual, and identity and relational conditions. The article further argues that adopting a framing that focuses on the public interest over professional self-interest helps to successfully overcome professional cleavages.