To protect themselves, democracies may restrict the rights of individuals or groups who seek to undermine this political system. However, despite a growing normative consensus about the legitimacy of such measures, public opinion on these restrictions remains largely underexplored. In this paper we focus on one of the most extreme manifestations of such militant restrictions on democracy: the banning of political parties. Specifically, we analyze whether and why citizens believe that democracies can legitimately ban certain political parties. To this end, we conducted an observational study and two survey experiments in 15 European democracies, complemented by an additional conjoint experiment in four of these countries. In these experiments, we manipulated: (i) the ideological orientation of parties promoting anti-democratic proposals; (ii) the intensity and nature of these proposals; and (iii) the popular support these parties enjoy. The findings reveal that Europeans are deeply divided over the legitimacy of banning political parties, with some citizens opposing such bans even for parties that openly advocate for overthrowing democracy. The results also reveal that these opinions are mainly influenced by citizens’ own partisan biases and their perceptions of the “real democratic threat” posed by these parties. These findings have relevant implications for ongoing debates about the legitimacy of militant democracy as a suitable approach to prevent and respond to democratic backsliding.