The 2020 U.S. presidential election saw rising political tensions among ordinary voters and political elites, with fears of election violence culminating in the January 6 riot. We hypothesized that the 2020 election might have been traumatic for some voters, producing measurable symptoms of post-traumatic stress disorder (PTSD). We also hypothesized that negative sentiment toward the opposing party correlates with PTSD. We measured PTSD using a modified version of the PCL-5, a validated PTSD screener, for 573 individuals from a nationally representative YouGov sample. We modeled the association between affective polarization and PTSD, controlling for political, demographic, and psychological traits. We estimate that 12.5% of American adults (95% CI: 9.2% to 15.9%) experienced election-related PTSD, far higher than the annual PTSD prevalence of 3.5%. Additionally, negativity toward opposing partisans correlated with PTSD symptoms. These findings highlight a potential need to support Americans affected by election-related trauma.