ABSTRACT The origin of the union ESK (Ezker Sindikalaren Konbergentzia) took place in the context of the Spanish Democratic Transition in the Basque Country and Navarre, when many radical unions and political parties emerged and disappeared. Its survival may help us trace better the effectiveness of union renewal strategies. Characterized by high conflict and mobilization rates at the company level, ESK militants sought to promote direct action and horizontal organizational structures standing at the opposite end of the model promoted by the new brand CCOO (Comisiones Obreras). The origin and development of ESK cannot be understood without the parallel analysis of the communist political party EMK (Euskadiko Mugimendu Komunista). ESK turned out to be the most effective and long-lasting materialization of EMK’s strategy of mass politics, and of what we conceive as a form of ‘integral militancy’ that is reflected in the immersion of union activists in ecologist, feminist, internationalist and anti-NATO struggles among others. EMK had a significant role in the promotion of unitary candidacies during the late 1970s and early 1980s, becoming the consolidation of a non-independentist non-centralized radical union model that continues to be referential among Basque struggles today.