This paper focuses upon a hybrid `performance hypothesis' positing that an established party will change its ideological identity in reaction to a successful new party only when the established party itself experiences poor election results which it can attribute to the new party. That hypothesis is addressed with original, longitudinal data on manifesto positions of the far-right Norwegian and Danish Progress parties, their Conservative neighbor-parties and (for control purposes) the Labour parties. Analyses support the authors' conclusion that both new parties have had a significant impact on their respective party systems according to the intentions and strategies of their founders.