Liberal democracy is being challenged by non-liberal democratic parties (NLDPs). The literature on
‘protecting democracy’ discusses what kind of legal, cultural, socio-economic and political measures are
right for countering NLDPs. This article focuses on political measures such as exclusion, collaboration and
policy co-optation, which seem particularly promising because in contrast to other measures, they do not
rely on state coercion but on the voluntary reactions of mainstream parties to their political peers. Still,
collaboration and co-optation may involve compromises with core principles of liberal democracy. Based
on the ethics of compromise, the article therefore asks to what extent it is permissible and obligatory for
mainstream parties to venture into compromises in order to safeguard liberal democratic principles. It
investigates three scenarios, one in which NLDPs are in opposition and two in which they are in
government and, respectively, have/have not (yet) altered liberal democratic institutions. It argues that the
space for compromise is larger the greater the threat is to the latter, but also that certainty about the effects
of compromise is essential to their permissibility. Where certainty is lacking, the default political measure
should be exclusion due to the moral loss and responsibility involved in making compromises.