President Donald Trump has refused to take part in a virtual TV debate with his Democratic rival Joe Biden.
The commission deciding the format of the 15 October debate in Miami said it would have to take place virtually.
It made the decision after Trump was diagnosed with coronavirus. He was treated in hospital, discharged and has no current symptoms, his doctor says.
Mr Trump said he would not waste his time on a virtual debate he said had been ordered to "protect" his rival.
The first debate descended into insults and interruptions. US media used words like "chaos" and "ugly" to describe it.
The vice-presidential debate, held on Wednesday night between Mike Pence and Kamala Harris, was a far more measured affair.
The next 90-minute presidential event is due to be held at the Adrienne Arsht Center for the Performing Arts in Miami, with local residents in the audience posing questions to the candidates.
But the commission said candidates would have to take part "from separate remote locations".
Participants and the moderator will be in Miami.
The commission said the decision had been made "to protect the health and safety of all involved".
Trump previously said he looked forward to debating Mr Biden on stage and on Monday Mr Biden said he would share a stage with Mr Trump as long as medical experts gave the go-ahead.
