Mainstream party reactions to the rise and persistence of populism have been rather peculiar in Finland. Contrary to other countries in Western Europe, where mainstream political parties have increasingly opened the door to cooperation with previously isolated populist parties, the Finnish case is about the mainstream moving from cooperation to the isolation of populist competitors. Indeed, soon after its parliamentary breakthrough in 2011, The Finns Party (PS), a populist radical right party, was involved in negotiations for the formation of the executive. Only after a brief period of governmental collaboration between 2015 and 2017, did mainstream centre-right parties turn to a marginalization strategy. This chapter assesses the causes and effects of the PS’s success. In particular, it considers how the party impacted on the dimensions of party competition and how mainstream parties responded to its challenge by adopting more ‘libertarian’ or ‘authoritarian’ positions. The chapter also includes a section focusing on how the PS interacted with a new populist party, Blue Reform (SIN), which was formed after its split in 2017.