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 cooptation 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.