Joe Biden and Kamala Harris launched their joint bid for the White House on Wednesday, with Ms Harris saying Americans are "crying out for leadership" to overcome a triple crisis of health, racial injustice and a ravaged economy.
They made their first appearance as running mates, before a small crowd of reporters and staffers at a Delaware high school closed to the public because of coronavirus concerns.
The Democratic duo pledged to end Donald Trump's presidency and, in Mr Biden's words, "rebuild this country".
"Everything we care about," said Ms Harris, 55. "It's all on the line."
A formidable US senator from California and daughter of immigrants, she has already made history as the first woman of colour chosen for a major party presidential ticket.
Who is Kamala Harris?
Ms Harris said the country was at a crossroads and left little doubt of her determination to go on the attack.
She said President Donald Trump "just isn't up for the job" and has left the nation "in tatters".
"America is crying out for leadership, yet we have a president who cares more about himself than the people who elected him," Ms Harris said.
Despite the awkwardly intimate setting, a far cry from the typical campaign rollout featuring an audience of thousands, Ms Harris appeared telegenic and confident in her first speech as vice presidential nominee.
She and Mr Biden arrived wearing face masks.
"The case against Donald Trump and Mike Pence is open and shut," the former Californian attorney general said.
Ms Harris assailed the president's "failures of leadership" on containing the deadly pandemic and reviving a hobbled economy.
The daughter of Indian and Jamaican immigrants to the US sought to tap into the nationwide upheaval over police violence and racial disparities.
"We're experiencing a moral reckoning with racism and systemic injustice that has brought a new coalition of conscience to the streets of our country demanding change," Ms Harris said.
Mr Biden's announcement of his running mate on Tuesday marked a crucial moment as he aims to build a broad coalition of voters to defeat the incumbent president.
He leads Mr Trump in most national polls and in swing states such as Florida and Wisconsin, despite conducting the campaign mainly from his Wilmington home.
Mr Biden said he chose Ms Harris as his partner because she was a tough and experienced legislator who was "ready to do this job on Day One".
Ms Harris's story as the child of immigrants and a woman of colour who fought her way to the top ranks of US legal and political circles would inspire the country, he said.
"Her story's America's story," Mr Biden said. "She's worked hard. She's never backed down from a challenge."
He spoke of optimism about the future, promising that their administration would do what it took to "turn the corner on this pandemic".
The Biden-Harris ticket already strikes a contrast with 2016, when nominee Hillary Clinton and running mate Tim Kaine struggled to unite a Democratic Party weakened by infighting between her supporters and those of progressive candidate Bernie Sanders.
Ms Harris's politics are to the left of Mr Biden's, but the pair have made a point of highlighting their shared values and goals despite their clashes during the primaries last year.
She is the first Indian American to be a presidential running mate, personifying the diversity considered key to building enthusiasm for the Democratic ticket.
Black politicians and activists were largely thrilled with the pick.
Ms Harris, who is two decades younger than Mr Biden and Mr Trump, could appeal to younger voters and women, particularly those in the suburbs who have lost faith in the Republican president, according to polling.
Mr Trump, who last month acknowledged Ms Harris would be a "fine choice" for Mr Biden, attacked her after Tuesday's announcement as "nasty" and a radical leftist who would implement "socialised medicine" and confiscate Americans' guns.
Mr Biden fired back at the president's attacks, asking if anyone was "surprised Donald Trump has a problem with a strong woman?"
Veteran political analyst David Axelrod put it more bluntly.
"If I were watching this Kamala Harris rollout in the White House, I'd be deeply concerned," Mr Axelrod said on Twitter.
"She nailed it. And them. Not 'nasty'. Powerful."
For Ms Harris, being chosen for the presidential ticket is political gold.
If she and Mr Biden win, she would almost certainly become the 2024 or 2028 Democratic presidential front-runner.
Mr Biden would be the oldest person to take the office, and there is broad speculation that he would serve one term and prepare his deputy to succeed him.
