Numerous polls suggest that COVID-19 is a profoundly partisan issue in the U.S. Using the geotracking data of 15 million smartphones per day, we found that U.S. counties that voted for Donald Trump (Republican) over Hillary Clinton (Democrat) in the 2016 presidential election exhibited 14% less physical distancing between March and May 2020. Partisanship was more strongly associated with physical distancing than numerous factors, including counties’ median income, COVID-19 cases, population density, and racial and age demographics. Contrary to our predictions, the observed partisan gap strengthened over time and remained when stay-at-home orders were active. Additionally, county-level consumption of conservative media (Fox News) related to reduced physical distancing. Finally, the observed partisan differences in distancing were associated with subsequently higher COVID-19 infection and fatality growth rates in pro-Trump counties. Taken together, these data suggest that U.S. citizens’ responses to COVID-19 are subject to a deep—and consequential—partisan divide.