Video report by ITV News Washington Correspondent Robert Moore
Joe Biden has picked California Senator Kamala Harris as his vice-presidential running mate.
The announcement ends weeks of speculation, and Harris' historic nomination makes her the first Black woman to compete on a major party's presidential ticket.
Biden, who served two terms as Barack Obama's vice president, had promised to nominate a woman as his vice presidential pick back in March.
Mr Biden tweeted: "I have the great honor to announce that I’ve picked Kamala Harris — a fearless fighter for the little guy, and one of the country’s finest public servants — as my running mate.
"Back when Kamala was Attorney General, she worked closely with Beau. I watched as they took on the big banks, lifted up working people, and protected women and kids from abuse. I was proud then, and I'm proud now to have her as my partner in this campaign."
Harris, a 55-year-old first-term senator, is also one of the party’s most prominent figures and quickly became a top contender for the No. 2 spot after her own White House campaign ended.
The pair will hope to bring an end to Donald Trump's time in the White House after the Republican defeated Hilary Clinton in 2016.
Shortly after the announcement on Tuesday, Mr Trump tweeted a video attacking Ms Harris, claiming she was part of the "radical left" and that "Slow Joe and Phoney Kamala" were "perfect together, wrong for America".
The US presidential election comes at a time of turmoil in the US. There has been more than five million coronavirus cases and 160,000 deaths, rising unemployment and riots breaking out in cities across the country.
Racial tensions have been heightened by the death of George Floyd, with many demanding systematic change.
A woman has never served as president or vice president in the United States. Two women have been nominated as running mates on major party tickets: Democrat Geraldine Ferraro in 1984 and Republican Sarah Palin in 2008. Their party lost in the general election.
The vice presidential pick carries increased significance this year. If elected, Biden would be 78 when he’s inaugurated in January, the oldest man to ever assume the presidency.
He has spoken of himself as a transitional figure and hasn’t fully committed to seeking a second term in 2024. If he declines to do so, his running mate would likely become a front-runner for the nomination that year.
Born in Oakland to a Jamaican father and Indian mother, Harris won her first election in 2003 when she became San Francisco’s district attorney. In the role, she created a reentry program for low-level drug offenders and cracked down on student truancy.
She was elected California’s attorney general in 2010, the first woman and Black person to hold the job.