Democratic presidential candidate Joe Biden has named Kamala Harris as his running mate – the first black woman and South Asian American in the role.
Once a rival for the top job, the California senator of Indian-Jamaican heritage had long been considered the front-runner for the number two slot.
The former California attorney general has been urging police reform amid nationwide anti-racism protests.
Mr Biden will face President Donald Trump in the election on 3 November.
After Tuesday’s announcement, Ms Harris tweeted that Mr Biden “can unify the American people because he’s spent his life fighting for us. And as president, he’ll build an America that lives up to our ideals.
“I’m honored to join him as our party’s nominee for Vice President, and do what it takes to make him our Commander-in-Chief.”
Mr Biden and Ms Harris will deliver remarks in Wilmington, Delaware, on Wednesday afternoon on “working together to restore the soul of the nation and fight for working families to move the country forward”, the Biden campaign said.
At a White House news conference on Tuesday, Mr Trump, a Republican, said he was pleased with Mr Biden’s choice, adding that Ms Harris did “very, very poorly” in her effort to become the Democratic nominee.
Ms Harris will take part in a debate with Mr Trump’s running mate, Vice-President Mike Pence, on 7 October in Salt Lake City, Utah.
In last year’s race to be the Democratic nominee, Kamala Harris showed herself to be a forceful speaker, launching blistering attacks on Donald Trump.
The role of a vice-presidential running mate isn’t always clearly defined.
One of the traditional roles is to go on the offensive in exposing the opposition’s weaknesses, while the presidential nominee focuses on communicating the party’s message, says the BBC’s North America reporter Anthony Zurcher.
Constitutionally, the vice-president steps in to the top job should the president die or leave office during his or her term.
Mr Biden will turn 78 in November, meaning should he be elected he will be the oldest US president in history (Ronald Reagan was 77 when he left office).
His age means Mr Biden’s vice-presidential choice may come under extra scrutiny.
Only two other women have been nominated as vice-presidential candidates for a major party – Sarah Palin by the Republican party in 2008 and Geraldine Ferraro by the Democrats in 1984. Neither were on the winning ticket.
A woman of colour has never been appointed to a presidential ticket by either of the two main American political parties. No woman has won the US presidency either.
What did Biden say?
Mr Biden tweeted that he had “the great honour” to name Ms Harris as his number two.
He described her as “a fearless fighter for the little guy, and one of the country’s finest public servants”.
He noted how she had worked closely with his late son, Beau, when she was California’s attorney general.
“I watched as they took on the big banks, lifted up working people, and protected women and kids from abuse,” he tweeted.
“I was proud then, and I’m proud now to have her as my partner in this campaign.”
Mr Biden pledged in March to name a woman on the ticket. He had faced mounting calls to pick a black woman in recent months as the nation has been convulsed by social unrest over police brutality against African Americans, a key voting bloc for the Democratic Party.
Who is Kamala Harris?
Ms Harris, 55, dropped out of the presidential race in December after failing to make headway in her bid to win the Democratic nomination.
She repeatedly clashed with Mr Biden during the primary election debates, most notably criticising his praise for the “civil” working relationship he had with former senators who favoured racial segregation.
Ms Harris was born in Oakland, California, to two immigrant parents: an Indian-born mother and Jamaican-born father.
She went on to attend Howard University, one of the nation’s preeminent historically black colleges and universities. She has described her time there as among the most formative experiences of her life.