The Federalist Convention that takes place tomorrow in Barcelona will probably not have the historical dimension of the event with a similar name that took place in Philadelphia in 1787. Nevertheless, it would be a mistake to underestimate the relevance of the Barcelona meeting. Probably for the first time, the main think tanks of the European left (the German Ebert Foundation and the FEPS) organize a high profile event to discuss the relevance of the federalist ideas for the construction of a united Europe and for the solution of institutional and sovereignty problems that affect the internal organization of European member states. Although the word "federalism" resonates more positively in some European countries than in others, it is important that two former presidents of the European Parliament and the president of FEPS and former Italian Prime Minister, Massimo Dalema, support with their presence an effort at the same time to help create a narrative that supports a united Europe and helps member states to deal with their internal tensions. Historically, social democrats have not been at the front of European federalism, perhaps because the construction of welfare states was mainly a national task. However, with increasing capital mobility and the need to complete the eurozone with a fiscal and political union, a modern social democracy sees in its interest to build an institutional architecture that facilitates the solution of problems that go much beyond the current nation states. In the USA, it can be argued that the construction of federalism has made much progress but is still a work in progress more than two centuries after their convention. In Europe, we cannot afford not to accelerate. The alternative is a return to the ghosts of European fragmentation, perhaps this time behind the disguise of a post-modern populism.