Research shows that union inclusion is critical to resisting precariousness, yet the role of institutional power is not adequately addressed. Through an investigation of eight retailers in four countries, this study uniquely examines how inclusive union strategies, cost competition and institutional power interact in different ‘spheres of action’. In the product market sphere, unions struggle to prevent labour cost competition between firms from eroding working conditions. In the production sphere, unions struggle to prevent labour cost competition between workers in a single firm from eroding working conditions. This article finds that multi‐level sources of institutional power are a precursor to effective union inclusion and articulating action towards threats from cost competition. I thereby argue that union efforts to resist precarious work are contingent on access to power from institutions. The article concludes with reflections on how institutional power relates to other forms of power.