High-profile political endorsements by scientific publications have become common in recent years, raising concerns about backlash against the endorsing organizations and scientific expertise. In a preregistered large-sample controlled experiment, I randomly assigned participants to receive information about the endorsement of Joe Biden by the scientific journal Nature during the COVID-19 pandemic. The endorsement message caused large reductions in stated trust in Nature among Trump supporters. This distrust lowered the demand for COVID-related information provided by Nature, as evidenced by substantially reduced requests for Nature articles on vaccine efficacy when offered. The endorsement also reduced Trump supporters’ trust in scientists in general. The estimated effects on Biden supporters’ trust in Nature and scientists were positive, small and mostly statistically insignificant. I found little evidence that the endorsement changed views about Biden and Trump. These results suggest that political endorsement by scientific journals can undermine and polarize public confidence in the endorsing journals and the scientific community.