How does the success of a movement influence the most extreme fractions within it? We examine this question in the context of the ascent of PEGIDA, Germany's most prominent right-wing movement since World War II. We combine a difference in differences strategy with variation in local weather conditions to show that municipalities experience a surge in hate crimes following protest days with a larger number of participants. The effects are most pronounced for municipalities with higher social media penetration and a more economically vulnerable refugee population. We also document that large protests have substantial spill-over effects to other municipalities that propagate through social media networks rather than geographic proximity. Our findings suggest that a public display of support for a movement — measured in terms of protest attendance and favorable public perception — can reduce the social cost of radical behavior and embolden extremist factions within the movement.