European Union referendums invite national electorates to vote on transnational cooperation and regional integration, thereby creating tension between transnational ballot issues and domestic electoral mobilisation. Because of the tension, domestic political parties are forced to confront a two-dimensional political space in EU referendums. In the referendum-generated political space, unless integration issues are more salient than domestic concerns, intra-divided and inter-converged mainstream parties tend strategically to abstain from the campaigns. Yet, explicit inter-party collusion may allow the pro-integration mainstream to form a party cartel in EU referendums. Suggestive evidence is drawn from a case study of the two Irish referendums on the Nice Treaty. Based on a party-candidate survey, Irish parties are mapped onto a latent two-dimensional political space. The findings shed new light on the initial abstention of Irish mainstream parties in the first Nice campaign and their subsequent mobilisation in the second referendum.
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