When Biden announced his choice the previous day, Trump said Harris had been "nasty", "mean" and "disrespectful" to Biden during the Democratic debates and Supreme Court justice Brett Kavanaugh during his confirmation hearings.

Biden described Harris, who previously served as attorney-general of California, as "smart, tough, experienced" and a "proven fighter".

"She is ready to do this job on day one," Biden said.

Biden said he wanted Harris to serve the same role in his administration as he did for former president Barack Obama: to be the last person in the room before he made important decisions.

"I asked Kamala to be the last voice in the room," he said.

Harris said she was "incredibly honoured by this responsibility" of being on Biden's ticket, describing the November 3 election as a "moment of real consequence for America".

"I am ready to get to work," she said.

"Everything we care about, our economy, our health, our children, the kind of country we live in, it's all on the line. We're reeling from the worst public health crisis in a century. The President's mismanagement of the pandemic has plunging us into the worst economic crisis since the Great Depression."

Displaying the skills she honed as a prosecutor, Harris went on the attack against Trump's handling of the coronavirus pandemic and the economy.

"The case against Donald Trump and Mike Pence is open and shut," she said.

"This virus has impacted almost every country, but there’s a reason why it hit America worse than any other advanced nation. It’s because of Trump’s failure to take it seriously."

She continued: "He inherited the longest economic expansion in history from Barack Obama and Joe Biden. And then, like everything else he inherited, he ran it straight into the ground."

Because of social distancing protocols, Biden and Harris appeared in a largely empty gym rather than the packed arena of supporters that would normally be typical for such an event.