Combo photo shows former US Vice President Joe Biden (L) and US President Donald Trump attending their respective events on different occasions. Photo:Xinhua

US President Donald Trump and his Democratic rival Joe Biden traded tough blows Monday as the White House race entered its final stretch, with the Republican leader branding his opponent "stupid" - and the Democrat firing back that the president lacked the "guts" to tackle the COVID-19 pandemic.

As Biden and his running mate Kamala Harris took their campaign message to must-win swing states Pennsylvania and Wisconsin, the president convened a surprise news conference at the White House.

In a freewheeling and grievance-laden performance, Trump once more teased the possibility of a COVID-19 inoculation by Election Day - something experts say remains unlikely - and accused his opponents of playing politics with a vaccine after Harris said she would not take his word alone on its safety.

Touting an upswing in job creation - after tens of millions lost jobs - and claiming the US is turning the corner on the pandemic, he called Biden "stupid," saying he "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."

Labor Day traditionally kicks off the final sprint of the campaign, with less than two months until the November 3 election - but the rival campaigns have been knocked off stride by multiple layers of turmoil, from the pandemic to the struggling US economy to deep racial unrest.

Candidates who normally would be skipping daily from state to state to speak before big crowds are limiting their movements and doing much more virtually.

And the sometimes violent anti-racism protests and counter-protests - the latest a pro-Trump motorcade rumbling Monday on the outskirts of Portland - lend an explosive element to the campaign.

Biden headed Monday to the swing state of Pennsylvania, where he held a socially-distant meeting with union leaders before taking questions from members of the huge American Federation of Labor and Congress of Industrial Organizations (AFL-CIO) union at its headquarters. 

Addressing the event, Biden hit back at Trump, charging that "he didn't have the guts to take on COVID[-19]."

"We know he's been great for his rich friends, but he hasn't been so great for the rest of us," charged Biden, who went on to assail Trump over a report in The Atlantic magazine that he has disparaged the military and its veterans.

"He's downright un-American," Biden fumed.

Though Trump has dismissed the report as a "hoax," it appears to have hit a nerve following a poll showing his support below that for Biden among active duty personnel.

AFP