In this paper I first contribute to the clarification of the slippery concept of "anti-system party" in order to enhance its utility for the study of contemporary party systems. Having discussed and provided a new definition of the concept of anti-system party after a conceptual reconstruction of the most used ‘anti’ labels adopted in the literature (e.g. anti-establishment, populist, protest party) I investigate the conditions favoring the electoral consolidation of anti-system parties in a critical yet decisive stage of development: the two elections after the first parliamentary breakthrough. Such a study aims to fill a substantial theoretical and empirical gap as the existing literature has paid very limited attention to the electoral fate of newcomers after their entry in the parliamentary arena. In particular, the conditions affecting the electoral trajectories in the medium to long term of 92 Western European anti-system parties in the period 1959-2014 are analyzed through the application of the Qualitative Comparative Analysis (QCA).