The problem of cross-case influences is crucial in the analysis of social phenomena. Is a referendum held in a state entirely a ‘domestic’ event? No work has applied diffusion theories to the study of referendum campaigns. In this paper, I show diffusion effects among the 2005 Constitutional Treaty referendums. Spain, France, the Netherlands, and Luxembourg used the referendum method to ratify the European Constitution. Based on 85 interviews with campaigners in all four countries, I find that campaign arguments and strategies were not always homegrown. However, such diffusion is not automatic and depends on diffusion channels.