This article addresses the question: under what conditions do democracies ban political parties? It does so by testing three hypotheses generated by a disparate literature on party bans in a 'most likely case', namely, the proscription of radical Basque nationalist parties Herri Batasuna, Euskal Herritarrok and Batasuna in 2003. These parties were banned for their integration in a terrorist network led by Euskadi Ta Askatasuna (ETA). The hypotheses are that democracies ban anti-system parties if (i) anti-system parties do not unambiguously eschew violence; (ii) alternatives to proscription are not effective; and (iii) relevant office holders have reason to believe they will not be disadvantaged in their pursuit of office or votes for supporting the ban. Case study findings confirm the hypotheses other than that on violence, given that ETA's political wing escaped proscription for around two decades before it was banned. Explaining this finding - or addressing the question of why the parties were not banned until 2003 the article develops two further hypotheses, namely, that democracies ban anti-system parties if (iv) the parties have been 'securitized' as an existential threat to the state or democratic community and (v) proscription is the preference of all veto players.
