Video report by ITV News Washington Correspondent Robert Moore
The former rivals for the Democrats’ presidential nomination sought to solidify their advantage over US President Donald Trump and secure their place in American history.
Mr Biden, a 77-year-old white man, embraced the significance of naming the first black woman to a major party’s presidential ticket, but he focused on other attributes Ms Harris brings to the ticket.
He hailed the California senator, the 55-year-old former prosecutor who a year ago excoriated Mr Biden on a primary debate stage, as the right woman to help him defeat Mr Trump and then lead a nation facing a pandemic, a wounded economy and a long-simmering reckoning with systemic racism.
At a high school gymnasium in his hometown of Wilmington, Delaware, Mr Biden said Ms Harris is “smart, she’s tough, she’s experienced, she’s a proven fighter for the backbone of this country”.
“Kamala knows how to govern. She knows how to make the hard calls. She’s ready to do this job on day one,” he continued.
Mr Biden spoke of her experience questioning Trump administration officials in the US Senate, and highlighted the historic nature of her pick, noting she is the daughter of immigrants from India and Jamaica.
“This morning, all across the nation, little girls woke up – especially little black and brown girls, who so often feel overlooked and undervalued in their communities. But today, today, just maybe, they’re seeing themselves for the first time in a new way,” Mr Biden said.
Ms Harris sat feet away from Mr Biden, listening with her mask off.
Taking to the stage after him, Ms Harris said she was “mindful of all the ambitious women before me, whose sacrifice, determination and resilience makes my presence here today even possible.”
She then launched into an attack on Mr Trump, lambasting him for a lack of leadership on the coronavirus pandemic.
“This is a moment of real consequence for America. Everything we care about – our economy, our health, our children, the kind of country we live in – it’s all on the line,” she said.
The event began an hour late. A momentous candidacy aside, the surreal nature of the scene was not only a woman of colour stepping into the role of would-be national executive but doing so in a mostly empty high school gym.
Masked reporters nearly outnumbered campaign aides and the candidates’ family members in a grim reminder of a coronavirus pandemic that has killed 165,000 Americans, while yielding Depression-level unemployment and Second World War-level national deficits.
In any other presidential election, vice-presidential picks are greeted with adoring throngs, and Ms Harris, given her profile, almost certainly could have expected an even more enthusiastic welcome to the ticket. On Wednesday, about 100 supporters stood outside the gymnasium waving signs prior to her arrival, with no hopes of being allowed inside.
The event was the first in a roll-out that Mr Biden’s aides say blends the historic nature of Ms Harris’s selection with the realities of the 2020 campaign and the gravity of the nation’s circumstances.
Later on Wednesday, the pair will lead an online grassroots fundraiser; they will continue campaigning together through the Democrats’ virtual convention that runs from next Monday to Thursday.