Donald Trump and Joe Biden have been trading insults over each other's position on a vaccine for Covid-19.
President Trump again hinted that a vaccine might be available before the November presidential election and accused his Democratic rivals of "reckless anti-vaccine rhetoric".
Mr Biden expressed scepticism that Mr Trump would listen to the scientists and implement a transparent process.
The US has six million cases of coronavirus, the highest in the world.
The virus has also claimed nearly 190,000 lives and fuelled a major recession, double-digit unemployment and sagging consumer confidence.
Last week it emerged the US Centers for Disease Control and Prevention (CDC) had urged states to consider "waiving requirements" in order to be able distribute a vaccine by 1 November – two days before the 3 November election.
No vaccine has yet completed clinical trials, leading some scientists to fear politics rather than health and safety is driving the push for a vaccine.
Both Mr Biden and his running mate Kamala Harris have questioned the president's credibility on the issue. Ms Harris said on Sunday she would not trust Mr Trump's word that a vaccine was safe, and Mr Biden also questioned whether the wider public would trust him too.
"He has said so many things that aren't true I am worried that if we do have a really good vaccine people are going to be reluctant to take it," Mr Biden said in Pennsylvania on Monday, Labour Day.
But he added that: "If I could get a vaccine tomorrow, I'd do it. If it cost me the election I would do it. We need a vaccine and we need it now. We have to listen to the scientists."
Mr Trump, who is trailing in the polls, hit back at a White House news conference, calling Mr Biden "stupid" and Ms Harris "the most liberal person in Congress… not a competent person in my opinion".
He said they "would destroy this country and would destroy this economy", and added that they "should immediately apologise for the reckless anti-vaccine rhetoric that they are talking right now".
The president, at times asking journalists to take off their face masks when asking questions, again suggested a vaccine could be ready next month. "We're going to have a vaccine very soon, maybe even before a very special date."
Mr Trump wants to have 300 million doses of a coronavirus vaccine in stock by January, and has spent hundreds of billions of dollars in tRead More – Source