Recent work finds that nonviolent resistance by minority ethnic groups is perceived as more violent and requiring more repression than nonviolent resistance by ethnic majorities, reducing its impact and effectiveness. We ask whether allies -- committed participants from majority groups -- can mitigate these barriers, and at what cost to the movements' core constituencies. We study this question in the context of Black Lives Matter, a movement that, in the summer of 2020, attracted unprecedented numbers of white participants. Employing an original survey experiment, we find that sizeable white presence at racial justice protests increases public approval for the protest, reduces perceptions of protester violence and reduces support for protest policing. We complement our experimental results with observational analysis of protest imagery shared in hundreds of thousands of tweets posted during the 2020 BLM protests. We find a substantial and significant association between the presence of whites in these images and user engagement, amplification, and positive sentiment. Taken together, these findings suggest that ally participation can be a powerful tool for countering the structural biases and obstacles
facing minority group movements fighting for social and political change.