Democratic presidential nominee Joe Biden (left) and US President Donald Trump. Photo: Xinhua

US Democrats on Tuesday nominated Joe Biden as their 2020 presidential candidate, formally designating the Washington veteran as the party's challenger to incumbent President Donald Trump in the November election.

In an unprecedented roll call vote that took place entirely online due to the coronavirus pandemic, all 50 states and seven territories announced their vote tallies that cemented Biden's role as the party flagbearer.

"Well thank you very, very much, from the bottom of my heart," a beaming Biden said in a live video link as he celebrated the nomination.

"It means the world to me and my family," he added, reminding viewers he will deliver a formal acceptance speech on Thursday at the conclusion of the four-day jamboree.

The nomination was a formality as he had already won the majority of the more than 3,900 delegates back in June.

The roll call came on Day 2 of the mostly virtual Democratic National Convention aimed at celebrating the party's candidate and welcoming independents and frustrated Republicans into their political movement to oust Trump from the White House.

The proceedings included a series of presentations by party leaders past and future who unleashed their own arguments against the White House incumbent and urged voters to rally around Biden.

The lineup featured president Jimmy Carter, who served one term from 1977, and 1990s commander-in-chief Bill Clinton, who warned that the Trump White House is swirling with chaos instead of the competence necessary to address the nation's crises.

"At a time like this, the Oval Office should be a command center. Instead, it's a storm center. There's only chaos," Clinton said.

"Just one thing never changes - his determination to deny responsibility and shift the blame," Clinton said.

The president faced a barrage of criticism on opening night too, most notably from former first lady Michelle Obama saying Trump lacks the character and skills for the job.

"I thought it was a very divisive speech, extremely divisive," Trump told reporters.

In her pretaped remarks to the convention, Obama urged Americans to rally behind Biden, her husband's former vice president, in the November 3 presidential election.

Trump's White House is marred by "chaos, division and a total and utter lack of empathy," Barack Obama's wife said in unprecedented criticism of a sitting US president by a former first lady.

"Donald Trump is the wrong president for our country," she added. "He is clearly in over his head."

AFP