Drawing on 61 interviews with Mexican immigrants and ethnographic participant observation conducted over three years, I compare social movement organizing in two cities in one California County: one more progressive and the other more repressive. I profile two campaigns waged by Mexican immigrants and their allies in response to two threats posed by police: (1) car impoundments of undocumented, unlicensed drivers’ vehicles and (2) police killings. As political process theory was extended to authoritarian settings, scholars have demonstrated that both growing political opportunity and threat stimulate mobilization. Building on this trend in the literature, this study’s contribution lies in its specification of the relationship among political opportunities, threat, and mobilization tactics. I argue increasing local political opportunity gives rise to more collaborative protest tactics, while relatively more threatening environments yield more confrontational tactics. Because opportunity and threat are not objectively assessed, nor do they automatically inspire protest, I also consider the role of state targets, formalized SMOs, and the influence of coalition partners on tactics. Ethnographic methods are particularly useful for understanding the way organizers and activists, from within organizations that favor distinct tactical repertoires, perceive and attribute threat and opportunity, shedding light on the micro- and meso-level dynamics that shape the social form of mobilization.