This paper examines whether protest associated with the “long protest wave” of the 1960s and 1970s positively influenced private-sector union support. Past research has found no such influence. We use measures designed to more closely represent the theoretical mechanisms proposed for spillover—specifically, union support rather than overall union density—and find a clear effect of protest on union support. Moreover, we find that this effect is greatest for unions with more progressive track records than for the labor movement as a whole. These results have implications for the social-movement literature on spillovers, labor studies, and organizational scholarship.