Original Article The role of Europarties in EU treaty reform: Theory and practice Karl Magnus Johansson School of Social Sciences, So ¨derto ¨rn University, 141 89 Huddinge, Sweden. E-mail: karl.magnus.johansson@sh.se Abstract This article examines the role of Europarties in the European Union (EU) institutional and constitutional or treaty reform, in decisions and negotiations leading to the adoption of treaties in the 1980s and 1990s. The existing literature on such reform in the EU largely overlooks the role of Europarties in the making of new treaties. Research on EU treaty reform usually operates within a state-centric ontology and framework for analysis. Challenging previous analyses and moving beyond state-centrism and inter- governmentalism, strictly inter-state bargaining, this article offers a complementary transnationalist account of what is happening in the drama of grand bargains or history- making treaty negotiations in the EU. There is a transnational dimension to such treaty reform; there is Europarty mobilization and influence. In conclusion, Europarties matter when they are in numerical ascendance, relatively cohesive and able to mobilize their networks of political parties and leaders. Acta Politica (2017) 52, 286–305. doi:10.1057/s41269-016-0024-y; advance online publication 25 October 2016 Keywords: Europarties; European People’s Party; European Union; Intergovernmental Conference; transnational; treaty reform Introduction In this article, I claim that an important part of the explanation for treaty reform in the European Union (EU) is Europarty input. 1 Among these, the centre-right European People’s Party (EPP) is the dominant one along with the centre-left Party of European Socialists (PES). Key players in and across these two Europarties have contributed to the fundamental change in the EU political and institutional environment, in which the Europarties themselves exist. There has been a resurgence of interest in Europarties, as they have become increasingly institutionalized and sought to raise their profile within the institu- tional framework of the EU (e.g., Day and Shaw, 2006; Hanley, 2008; Ku ¨lahci, Ó 2016 Macmillan Publishers Ltd. 0001-6810 Acta Politica Vol. 52, 3, 286–305 www.palgrave.com/journals