Can extremist parties benefit from a backlash against European Union integration? A theoretical model that integrates demand-side, supply-side and political opportunity space explanations for extremist party success is used here in an effort to predict the conditions under which extremist parties may have utilized increased public discontent with EU integration to increase their vote share in national legislative elections. The plausibility of the model is then tested against the evidence in 14 EU member states from 1992 to 2006, with the use of matrices and political opportunity space maps. In the majority of the elections examined, extremist parties increased their vote share in circumstances fitting the theoretical model.