Environmental damage. The Trump administration has canceled green energy projects (wasting hundreds of millions of previously invested funding in the process), supported fossil fuel companies, removed protections from federal land, defunded environmental regulatory agencies, and disabled climate monitoring conducted by NOAA, NASA and NWS.

Above and beyond everything else that Trump is responsible for, the environmental damage is going to be affecting not just US citizens but the entire world far into the future. Nothing else will compare to the scale of human suffering and death that will be caused by the Trump administration’s environmental policies.

The other things that have been mentioned are more immediate and visceral and tend to provoke stronger emotional responses, but the environmental damage is going to last much longer, be much more difficult to fix, and kill many many more people.