Donald Trump and his presidential rival Joe Biden traded blows on Monday as the White House race entered its final stretch.
The Republican leader called his opponent "stupid", and the Democrat responded that Mr Trump lacked the courage to tackle the Covid-19 pandemic.
As Mr Biden and his running mate Kamala Harris took their campaign message to must-win swing states Pennsylvania and Wisconsin, the incumbent called a surprise briefing.
Mr Trump again hinted at the possibility of a Covid-19 inoculation by election day on November 3, which experts say is unlikely.
He accused his opponents of playing politics with a vaccine after Ms Harris said she would not take his word alone for its safety.
Mr Trump told of a rise in job creation, after tens of millions lost their employment, and claimed the US was turning the corner on the pandemic.
He said Mr Biden "wants to surrender our country to the virus, he wants to surrender our families to the violent left-wing mob and he wants to surrender our jobs to China".
Labour Day is traditionally the start to the final sprint of the campaign, with less than two months until the election.
But the rival campaigns have been knocked off stride by layers of turmoil, from the pandemic to the struggling US economy to deep racial unrest.
Candidates who would normally would be travelling daily from state to state to speak before big crowds are limiting their movement and doing much more online.
And the sometimes violent anti-racism protests and counter-protests lend an explosive element to the campaign.
The latest was a pro-Trump motorcade rumbling on Monday on the outskirts of Portland.
Mr Biden flew to the swing state of Pennsylvania, where he held a meeting with union leaders before taking questions from the American Federation of Labor and Congress of Industrial Organisations at its headquarters.
Addressing the event, he said Mr Trump "didn't have the guts to take on Covid".
"We know he's been great for his rich friends but he hasn't been so great for the rest of us," Mr Biden said.
He then attacked Mr Trump over a report in The Atlantic magazine that he has disparaged the military and its veterans.
"He's downright un-American," Mr Biden said.
Mr Trump dismissed the report as a hoax but it appears to have hit a nerve after a poll showed Mr Biden led for support among active duty personnel.
"I'm not saying the military [leadership] is in love with me," he said. "The soldiers are."
Mr Biden, 77, last week picked up the pace of campaigning but said the coronavirus has made him more cautious than Mr Trump, 74, who has appeared before hundreds of supporters.
But polls show Mr Biden maintaining a lead over Mr Trump, with both increasingly focusing on key upper Midwest states such as Wisconsin, where polling is closer, and where Hillary Clinton narrowly lost to the president in a 2016 shock.
In her highest-profile campaign trip yet, Ms Harris travelled to Wisconson where, like Mr Biden, she met the family of Jacob Blake, the African American man whose shooting by police caused widespread protests last month.
The California senator, who is the first woman of colour on a major-party presidential ticket, spoke by phone to Mr Blake as he recovered in hospital.
Ms Harris later met union members and black businesspeople.
Black participation dropped in Wisconsin in 2016, and could prove pivotal this year.
Mr Trump's running mate, Mike Pence, also went to Wisconsin on Monday, to speak to an energy co-operative in the western city of La Crosse.