Kamala Harris has been described as 'the obvious pick' for Joe Biden's running mate against US President Donald Trump.
Mr Biden announced her selection on Tuesday as his nominee for US vice-president on the Democratic Party ticket.
She becomes the first black woman on a major presidential ticket in US history.
Ms Harris tweeted on Tuesday: "@JoeBiden 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."
.@JoeBiden 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.
— Kamala Harris (@KamalaHarris) August 11, 2020
In choosing Ms Harris, Mr Biden is embracing a former rival from the Democratic primary who is familiar with the pressures of a national campaign.
Ms Harris' presidential campaign collapsed months ago amid questions over her political identity.
Mr Biden had been pressed to select a woman of colour as his running mate as the US is in the midst of a reckoning over its history of racial injustice.
Ms Harris, who became just the second black woman elected to the Senate, is one of the Democratic Party's most prominent figures.
— Joe Biden (@JoeBiden) August 12, 2020
At the regular White House news conference, Mr Trump expressed his surprise at the choice - saying Ms Harris had been "very, very nasty" to Mr Biden during the Democratic Party primaries.
"One of the reasons that it surprised me, she was probably nastier than even Pocahontas to Joe Biden," he said.
"She was very disrespectful to Joe Biden and it's hard to pick somebody that's that disrespectful."
A woman has never served as president or vice president in the United States.
'The obvious pick'
Anthony Zurcher, the BBC's North America correspondent, told Newstalk Breakfast: "She had been the obvious pick for a while - and that's because first of all she is younger than Joe Biden, Joe Biden's 77-years-old, she's in her 50s - so it balances the ticket that way.
"Also, to put it bluntly, Joe Biden's an old white man - Kamala Harris is a woman of multi-ethnic descent, her father was from Jamaica, her mother was from India.
"She went to an historical all-black college, Howard University, so it beings diversity to the ticket that reflects the diversity in the Democratic Party.
"And I think another reason why he picked her, as compared to some of the other candidates who are also black women who were out there, was that she had run for president in 2019.
"She'd kind of been through that pressure cooker of national politics, she served in the Senate, she ran for state-wide office in California.
"So she kind of fits the role of being someone who is ready for the spotlight and ready, conceivably, to take over the presidency if something were to happen after Joe Biden got elected".
Asked about her support by others in the party, he said there were initially some doubts from those on the left.
But he added: "When I talk to progressives right now, they just want to beat Donald Trump.
"And so I think they're going to bite their tongues by-and-large and support Kamala Harris, and support Joe Biden, because the alternative Donald Trump is just too much of a pill for them to swallow."
On Mr Trump, he said: "They've already started to release videos, he had comments about her during his press conference on Tuesday, he said that she was a 'nasty woman'.
"That she was particularly harsh on her hearing that she was in, considering one of this Supreme Court nominees - Brett Kavanaugh.
"The video attacked her as being too far left, as being inauthentic, as being kind of wishy-washy and insincere.
"But it fits in with the attacks on Joe Biden in general, which is that he's just a vessel for the far-left of the Democratic Party.
"And that even if he's perceived as a moderate, what's really going to happen is people are going to pull him to the left".
Who is Kamala Harris?
Although a first-time senator, the 55-year-old quickly became a top contender for the number two spot after running her own White House campaign.
Ms Harris has declared herself a "progressive prosecutor" who backs law enforcement reforms.
Born in Oakland to a Jamaican father and Indian mother, Ms Harris won her first election in 2003 when she became San Francisco's district attorney.
In that time, she created a re-entry programme for low-level drug offenders and cracked down on student truancy.
She was elected California's attorney-general in 2010, making her the first woman and black person to hold the job.
In 2016, she was elected to the US Senate and built a reputation around her work as a prosecutor.
Ms Harris gained attention for her forensic questioning of Trump administration officials and launched her presidential campaign early last year with the slogan "Kamala Harris For the People", referencing her work in the courtroom.
Additional reporting: IRN