In many countries, unions, with conflicting political identities, compete for works council positions. However, inter-union solidaristic forms of cooperation can occur within these same institutions. This article advances the concept of strategic group identity to explain the makeup, success, failure and longevity of inter-union cooperation in this context. Based on case studies of inter-union conflict and cooperation on works councils in the French automotive industry, the article highlights the importance of inter-union identity congruence in examining responses to shared threats. Several implications for understanding inter-union solidarity are developed.