Does racial diversity make forming a union harder? Case studies offer conflicting answers, and little large-scale research on the question exists. Most quantitative research on race and unionization has studied trends in membership rather than the outcome of specific organizing drives and has assumed that the main problem is mistrust between workers and unions, paying less attention, for example, to the role of employers. The author explores the role of racial and ethnic diversity in the outcomes of nearly 7,000 organizing drives launched between 1999 and 2008. By matching the National Labor Relations Board’s information on union activity with the Equal Employment Opportunity Commission’s surveys of large establishments, the author reconstructs the demographic composition of the work groups involved in each mobilization. The study finds that more diverse establishments are less likely to see successful organizing attempts. Little evidence is found, however, that this is because workers are less interested in voting for unions. Instead, the organizers of more diverse units are more likely to give up before such elections are held. Furthermore, this higher quit rate can be explained best by considering the other organizations involved in the organizing drive. In particular, employers are more likely to be charged with unfair labor practices when the unit in question is more racially diverse. This effect persists when the study controls for heterogeneity among industries, unions, and regions.