US President Donald Trump has refused to take part in a virtual TV debate with his Democratic rival Joe Biden.
Earlier the commission organising the 15 October debate in Miami said it would have to take place remotely.
It made the decision after Trump was treated for Covid-19. He has no current symptoms but the White House is tackling a cluster of positive tests.
He said the move to virtual was to "protect" his rival. Biden said Trump "changed his mind every second".
The first presidential debate on 30 September descended into insults and interruptions. The vice-presidential debate, held yesterday between Mike Pence and Kamala Harris, was a far more measured affair.
The US election will be held on 3 November. Latest opinion polls give Biden a high single digit lead nationally, but the outcome is often decided in battleground states where the races can be much closer.
Trump fears being cut off
His comments during a phone-in interview with Fox Business Channel touched on a number of key matters, including his health and the possibility of movement towards a stimulus package for the economy.
But it was his comments on the debate format that drew most attention.
Trump said: "I'm not gonna waste my time on a personal debate. Sit behind a computer, ridiculous. They cut you off... I'm not doing a virtual debate."
He also described the moderator of the Miami debate - political editor of the cable and satellite television network C-SPAN, Steve Scully - as a "never Trumper".
A statement from Trump campaign manager Bill Stepien, who had also previously tested positive for Covid, said the decision of the commission to "rush to Joe Biden's defence" was "pathetic" and Trump would have posted "multiple negative tests" before the debate.
He said Trump would hold a rally instead.
On his health, Trump said: "I'm back because I'm a perfect physical specimen."
He said he had stopped taking most "therapeutics" but was still taking steroids and would be tested for Covid again "soon".
But although his doctor has said he now has no symptoms, questions still remain about when the president first became infected and whether he could still be contagious.
And although the names of many people who have interacted with the president and tested positive are now known, it remains unclear just how many were exposed at the White House. New Covid safety measures are in place there.
Trump said that "somebody got in and people got infected" but gave no more details.
A gathering on 26 September announcing Trump's Supreme Court pick has been seen as a possible "super-spreader" event, with a number of attendees known to have tested positive.
What's the Biden team's position?
Biden had previously been wary of an on-stage event, saying he would attend if medical experts gave the go-ahead.
He said after Trump's comments on the virtual debate: "We don't know what the president's going to do. He changes his mind every second. For me to comment on that now would be irresponsible.
"I think I'm going to follow the commission's recommendation. If he goes off and he's going to have a rally I don't know what I'll do. You never know what's going to come out of his mouth."
Biden communications director Kate Bedingfield said: "Vice-President Biden looks forward to speaking directly to the American people and comparing his plan for bringing the country together and building back better with Donald Trump's failed leadership on the coronavirus that has thrown the strong economy he inherited into the worst downturn since the Great Depression."
Debate commission aiming to protect health
The 90-minute presidential debate was due to be held at the Adrienne Arsht Centre for the Performing Arts in Miami, with local residents in the audience posing questions to the candidates.
But the Commission on Presidential Debates said the candidates would have to take part "from separate remote locations". Participants and the moderator would be in Miami.
The commission said the decision had been made "to protect the health and safety of all involved".
One further debate is currently scheduled - on 22 October in Tennessee.
What about the economy?
The president on Tuesday had said he was breaking off talks with the Democrats on an overarching Covid stimulus package but, after a negative stock market reaction, said he would instead try to agree individual deals.
He was more bullish yesterday, saying he saw "really good" odds of reaching a deal on a range of issues.
"We're starting to have some very productive talks," he said.
The main areas of progress were on assistance for airlines and a $US1200 ($NZ1800) stimulus cheque for workers.
Republicans under Treasury Secretary Steven Mnuchin and Democrats under House Speaker Nancy Pelosi have been in regular discussions but have differed on the amount they want to spend.