How does partisan alignment with a populist leader shape compliance with public health measures? During Mexico’s COVID-19 campaign, the president publicly undermined preventive guidelines even as his administration promoted them. Using precinct-level mobility data and a novel shift-share IV, we show that support for the president’s party increased mobility, infections, and deaths. A calibrated epidemiological model indicates that partisan divergence explains most of the infection gap. The evidence supports a supply-side mechanism in which elite cues—rather than fixed partisan traits—shape compliance during crises. Our findings highlight how populist leaders influence public health through policy, symbolic, and rhetorical authority.