The implications of rising parliamentary representation of populist parties have been thoroughly studied but little is known about the impact of populist state leaders on party positions. In this article, we study mainstream parties' strategic responses when a populist takes over as the leader of a nation. We use content-analytical data and large language modelling to measure positions expressed in manifestos from parties from 51 democracies between 1989 and 2018. Employing methods for causal inference from observational data, we find that right-wing populist state leaders induce mainstream parties to differentiate their positions on multiculturalism, possibly leading to polarization of the party system. Under left-wing populist leaders, mainstream parties adopt more homogenous or differentiated positions, depending on the policy category and other contextual factors. Parties are generally more responsive in emerging than advanced countries and in presidential than parliamentary systems.
