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.

US President Donald Trump
US President Donald Trump speaks after stepping off Air Force One at Andrews Air Force Base in Maryland on September 3, 2020. Photo: AFP / MANDEL NGAN

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.

Joe Biden
Democratic presidential candidate Joe Biden headed to Pennsylvania as Labor Day kicks off the final stretch of the White House race. Photo: AFP / JIM WATSON

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 AFL-CIO union at its headquarters.

Addressing the event, he hit back at Trump, charging that "he 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," charged Biden, who went on to assail Trump over a report in The Atlantic magazine that he has disparaged the military and its veterans.

Kamala Harris
Democratic vice presidential nominee Kamala Harris arrives at Milwaukee Mitchell International Airport on September 7, 2020. Photo: AFP / Kerem Yucel

"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.

"I'm not saying the military (leadership) is in love with me -- the soldiers are," he told reporters at the White House.

Protests in Rochester, NY
Rochester, New York has seen five nights of protests over video evidence that shows the death of Daniel Prude, a black man with psychological problems, who was placed in a hood and forced face-down on a road by police. Photo: AFP / Maranie R. STAAB

At 77, Biden last week picked up the pace of campaigning but, citing the Covid-19 threat, has been far more cautious than Trump, who at 74 has appeared before hundreds of supporters.

Still, polls show Biden maintaining a persistent lead over Trump, with both increasingly focusing on key upper Midwest states like Wisconsin, where polling is closer -- and where Hillary Clinton narrowly lost to Trump in a 2016 shocker.

Harris, in her highest-profile campaign sortie yet, headed to the state where she followed in Biden's footsteps by meeting with the family of Jacob Blake, the African American man whose shooting by police touched off widespread protests last month.

The California senator -- the first woman of color on a major-party presidential ticket -- spoke by phone to Blake himself as he recovers in hospital. Later she met with union members and black businesspeople.

Black participation dropped in Wisconsin in 2016, and could prove pivotal this year.

Pence also headed to the state Monday to deliver remarks to an energy cooperative in the western city of La Crosse.

Up to now, the shadow of the coronavirus has produced a somewhat muted campaign, with Biden originally spending so much time in his Delaware home that Trump taunted him about coming out of "his basement."

But the sometimes violent anti-racism protests simmering around the nation have created a tinderbox situation.

Trump, blaming the violence on "radicals" and "anarchists," has positioned himself as a law-and-order champion.

Portland, where tensions are running high, was on edge Monday as hundreds of pro-Trump protesters -- some of them armed and with military gear -- were gathering for a motorcade around the city, which has seen months of unrest and two deaths among protesters on either side.

A sixth night of demonstrations was also expected in the New York city of Rochester, where a black man with psychological problems, Daniel Prude, died after being placed in a hood and forced face-down on a road by police.

Biden has issued a sharp rebuke of the violence and looting that have accompanied some protests, even while denouncing racism as the country's "original sin."

Copyright AFP. All rights reserved.