This article investigates the impact of niche party success on the policy agendas of mainstream parties. Following from the expected electoral effects of issue politicization, the success of radical right and green parties will cause different reactions from mainstream parties. While mainstream parties emphasize anti-immigrant positions in response to radical right success, green party success will have the opposite effect for environmental issues. Since green parties constitute issue owners, their success will make established parties de-emphasize the environment. Analyzing time-series cross-section data for sixteen Western European countries from 1980 to 2011, this article empirically establishes that green and radical right parties differ in their effect on mainstream party behavior and that their impact depends on the ideological position and past electoral performance of the mainstream parties.