What is the potential for European interest groups to jointly mobilize to promote synergies between ecological and social goals? This chapter explores attempts of the 'Right to Energy for All Europeans' coalition, an alliance of Brussels-based NGOs and trade unions, to exert influence on the energy poverty-related proposals in the 2016 Clean Energy for All Europeans legislative package. Drawing on qualitative research methods, we find that structural constraints (such as limited resources and compartmentalized policymaking) undermine full bottom-up eco-social coordination at the EU level. So does the commitment of each pressure group to potentially divergent interests. Nevertheless, European green and social NGOs and trade unions display a cooperative attitude towards each other and against the established constraints to coalition-building. Together they advocate for a just transition paradigm, combining social and environmental goals and ultimately making eco-social mobilization an efficient advocacy strategy.