US Democrats opened their unprecedented online nominating convention Monday with a show of unity behind Joe Biden, as former first lady Michelle Obama said the party's presidential candidate "knows what it takes" to lead the nation out of crisis.

With the Democratic Party poised to officially anoint the 77-year-old Biden as its nominee, President Donald Trump defied coronavirus concerns and staged competing events in Wisconsin and neighbouring Minnesota.

Joe Biden and Kamala Harris
Joe Biden and his newly named running mate Kamala Harris will be the focus of the Democrats' virtual convention starting August 17. Photo: AFP / MANDEL NGAN

But at 8:00 pm (0100 GMT Tuesday) Wisconsin time, Americans tuned in to what appeared to be a carefully choreographed opening for the four-day unifying confab.

"Every four years we come together to reaffirm our democracy," the convention's moderator, actress Eva Longoria, said in the opening moments.

"This year we've come to save it."

A sign advertises DNC
A sign advertises the Democratic National Convention in Milwaukee, Wisconsin, which was to have hosted the quadrennial event before the coronavirus pandemic stuck. Photo: AFP / Bryan R. Smith

Barack Obama's popular wife Michelle was given the primetime slot on the opening night of the convention, which was to have been held over four days in Milwaukee, Wisconsin, but which is now taking place almost entirely online because of the COVID-19 outbreak.

In excerpts of her remarks released ahead of her taped speech, Obama said Biden was a "terrific vice president" during the eight years he served as her husband's number two.

"I know Joe. He is a profoundly decent man guided by faith," she said.

Senator Kamala Harris
Senator Kamala Harris appears to be popular choice within the party as Biden's vice presidential pick. Photo: GETTY IMAGES NORTH AMERICA / Drew Angerer

"He knows what it takes to rescue an economy, beat back a pandemic and lead our country," the former first lady said.

"He will tell the truth, and trust science," she said in a jab at Trump, who has been accused of repeatedly ignoring the advice of his scientific advisors on how to respond to the pandemic.

Vermont Senator Bernie Sanders, who challenged Biden for the nomination from the progressive left, was also due to address the convention, which is being live-streamed, on Monday.

US president Donald Trump
U.S. President Donald Trump is pictured. Photo: AFP/JIM WATSON

"This election is the most important in the modern history of this country," Sanders said in prepared remarks.

"The future of our democracy is at stake," he said. "The future of our planet is at stake."

Electing Biden over Trump is an absolute necessity, he stressed. "My friends, the price of failure is just too great to imagine."

Democrats kick off convention
Democrats, forced to hold the 2020 nominating convention online, are looking for ways to replace some of the excitement seen in this file photo from the party's 2012 convention in Charlotte, North Carolina. Photo: AFP / Brendan SMIALOWSKI

Trump flew on Air Force One meanwhile to Mankato, Minnesota, and Oshkosh, Wisconsin and delivered remarks to supporters gathered on the airport tarmacs.

US President Donald Trump
IMAGES AND SOUNDBITESUS President Donald Trump claims that Michelle Obama and other Democrats due to speak at the Democratic National Convention (DNC) "are taping their speeches," while talking on the tarmac of Minneapolis-Saint Paul International Airport on his way to Wisconsin. Photo: DC POOL

Speaking in Oshkosh, Trump accused Biden and his running mate Kamala Harris of seeking to enact "crazy socialist policies."

"This is the most dangerous election we've ever had," Trump said. "This will be a very large-scale Venezuela if they win."

"The only way we're going to lose this election is if the election is rigged," added the president, who trails Biden in nearly all national polls as well as multiple battleground states.

The Democratic convention is taking place amid a furor over Trump's own efforts to limit mail-in voting.

Insisting without proof that it fosters fraud, Trump has threatened to block extra funding that Democrats say is urgently needed to allow the US Postal Service to process millions of ballots.

Democratic Party chairman Tom Perez said the convention would push back against Trump's "assault on our democracy" by highlighting efforts to protect voting by mail.

Oshkosh, where Trump was speaking, is about a 90-minute drive north of the Milwaukee arena where Democrats had intended to gather in a sign of eagerness to win back Wisconsin, one of multiple Democratic strongholds which flipped to Trump in 2016.

But the coronavirus pandemic, which has killed some 170,000 people in the United States, upended election campaigning.

The Democratic convention and the Republican event, which begins on August 24, will be held virtually, without the usual roaring crowds and confetti.

"We may be physically apart, but this week Democrats are coming together from across the nation to put forth our vision for a better America," Biden tweeted.

The experimental convention format will allow speakers to address American voters unfiltered -- largely shorn of the usual overwrought stagecraft and screaming delegates.

Biden and Harris will address the convention live via videolink, while other speeches will be "a mix of live and curated content," the campaign said.

Biden, whose poll leads over Trump remain significant, despite a slight tightening of the race, is hoping Harris -- the first woman of color on a major party's presidential ticket -- will invigorate Democrats.

The California senator, former prosecutor and daughter of Indian and Jamaican immigrants is 55 and brings relatively youthful energy to the ticket.

On Wednesday, Barack Obama will speak, and Harris will have her spotlight moment before the convention culminates Thursday when Biden formally accepts the Democratic nomination and delivers his acceptance speech.

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