This paper explores the potential for “environmental” (green) and “social” (red) European NGOs and trade unions to jointly mobilize, pursuing synergies between ecological/environmental and social goals, as well as to drive bottom-up policy change at the European Union level. The main research question is thus the following: do European NGOs and trade unions conflict or cooperate on ‘eco-social’ challenges? To answer such question, the research focuses on the case of the ‘Right to Energy for All Europeans’ coalition. The coalition is an advocacy-oriented alliance of European social and green NGOs, as well as trade unions, aimed at eradicating energy poverty in Europe, mostly by exerting influence EU’ institutions concerning the “Clean Energy for All Europeans” legislative package. From a methodological point of view, we rely on literature review and qualitative research methods, notably document analysis complemented by several semi-structured interviews with the members of the Coalition. We draw two main conclusions. First, bottom-up coordination is undermined by structural constraints (such as limited resources and compartmentalized policymaking), as well as by interest groups’ commitment to specific and potentially divergent interests. Indeed, coalition-building is always a strategic move and it is more likely to take the form of ad-hoc cooperation, than formalized coordination. Nevertheless, European green and social NGOs and trade unions display a cooperative attitude towards each other. This is coherent with our second finding, according to which these organizations have an incentive to cooperate, since they frequently endorse a ‘just transition’ paradigm. Contrasting both powerful economic interests and the approach followed by European institutions, NGOs and trade unions are promoting a vision for Europe where social and environmental goals are harmoniously combined. Green-red alliances can thus be seen as bottom-up actions aimed to effectively mainstream ‘just transition’ into policy demands and political strategies, hence giving their members an incentive to join.