In European politics and beyond, parties of the mainstream left and right must grapple with how to respond to the rise and consolidation of radical right parties (RRPs). A prominent debate in political science centers on the efficacy of `accommodative’ strategies for mainstream parties. However, existing evidence is weakened by endogeneity concerns and limited external validity. We contribute to this debate with a well-powered cross-country experimental design in four prominent cases (Denmark, Sweden, Spain, and Germany). Our design not only strengthens internal and external validity but also incorporates both mainstream left and right parties within a unified theoretical framework, and measures outcomes beyond vote choice, including the perceived legitimacy of RRPs. In doing so, we provide novel, comprehensive evidence of the trade-offs mainstream parties face when adopting accommodative strategies in multiparty systems.