Research on the European Citizens’ Initiative has neglected the crucial role of social movements. This article contributes to the study of mobilization strategies and campaign consequences through two novel arguments: First, in explaining successful mobilization, I argue that the European Citizens’ Initiative as a political opportunity structure incentivizes the organization of nationally focused campaigns centred in at least one large, that is, populous state, instead of Europe-wide activism. Second, in explaining the multiple consequences of European Citizens’ Initiative mobilization, I show that a campaign may have important, often unintended, effects beyond (the failure to achieve) policy change, including disappointment with the democratic process leading to European Citizens’ Initiative reform and a renewed focus on national politics. This article provides an in-depth analysis of one of the only five European Citizens’ Initiative campaigns that have been able to collect the required one million signatures, while referring to other campaigns as shadow cases. The findings have important implications for debates on EU democracy.