The billboard was positioned directly opposite Des Moines airport, where his hangar rally will be held.
Local officials were told to expect “up to 10,000 people at the rally,” according to the Des Moines Register.
Iowa Rural America 2020, an anti-Trump group of “farmers, agri-business professionals, former elected officials and community leaders” paid for the billboard as a warning.
Chris Henning, a member of the steering committee, said in a press release that everyone should “be worried about a President who was in the hospital with Covid last week and who now wants to pack thousands of Iowans into an airport hangar”.
“This is the height of irresponsibility. We saw what happened in the Rose Garden. Why should the President be allowed to bring that kind of super-spreader behaviour into Iowa, particularly when our cases are rising?” he said.
Covid is currently spiking in the state, which has so far seen 102,000 cases and 1,500 deaths.
Outbreaks are currently reported at 61 long-term care facilities; the highest number since the pandemic began, and a record 473 patients are in hospital with the virus, up from the previous record of 463 set on Tuesday.
On Monday night Dr Anthony Fauci warned that holding large rallies was “asking for trouble”, pointing to a rise in cases in several midwest states, in particular Iowa.
“If anyone in attendance is infectious, we are potentially looking at another super-spreader event,” said Lina Tucker Reinders, executive director of the Iowa Public Health Association.
She told the Des Moines Register: “We again today set a record high for hospitalisations. We need to be focusing on bringing those numbers down and controlling the spread, not enabling large events, political or otherwise.”