The Democrats hope the entry into the 2020 presidential race of the first black woman and Asian American increases their chances of beating Donald Trump.
Joe Biden's choice for running mate in the race for the White House is the first black woman and Asian American on a major presidential ticket in US history.
Kamala Harris was seen as a favourite for the nomination after the US Democratic presidential hopeful said in March he would choose a woman to be his potential vice-president.
The California Senator is a vocal critic of Donald Trump and her entry into the contest ahead of November's election has already brought an attack from the US president.
Harris joins the Democratic ticket at a complicated moment in the United States, which remains hard hit by the coronavirus pandemic and economic woes while facing protests over police violence and racial injustice.
Who is Kamala Harris and what does she stand for?
Kamala Harris was born in California to a Jamaican father and an Indian mother. After working as a county prosecutor focusing on sexual assault cases, she was elected San Francisco's district attorney in 2003 -- the first Asian American and first woman to hold the post.
Seven years later, she was voted in as the state of California's attorney general.
"I decided to become a prosecutor because I believed that there were vulnerable and voiceless people who deserved to have a voice in that system," she said.
Harris was elected to the US Senate in 2016, as California's first black Senator. As a member of four Senate committees, she gained a reputation as a tough questioner. no doubt helped by her background as a prosecutor.
Former Attorney General Jeff Sessions said during one exchange that Harris' questioning made him "nervous".
The 55-year-old launched her presidential campaign in 2019, as a candidate during the democratic primaries. But she withdrew from the race in December before the first votes were cast.
She is seen as a centrist and during the primaries tried to strike a balance on the issues of institutional racism and police brutality. Among her policy priorities are criminal justice reform and racial justice legislation.
How popular is Harris in Democratic circles?
"Together we will beat Donald Trump," headlined the Democrats' website profile of Harris as they launched the joint ticket with Biden.
Many see Harris' candidacy as an acknowledgement of the crucial role black voters are likely to play in the November election.
Former President Barack Obama said the appointment was "a good day for our country". Harris was "more than prepared for the job" and had "spent her career defending our Constitution and fighting for folks who need a fair shake," he tweeted.
Party strategist Christine Pelosi, who is also from California and knows Harris well, said she was "thrilled" at the appointment.
"Hearing from African-American friends and Indo-American friends all over the country, some of whom are just literally weeping with joy and recognition that they're finally seen and elevated, is really a special moment," she told the BBC.
But during Harris' primary campaign last year, her law enforcement background prompted scepticism from some progressives. Her record as a prosecutor was scrutinised and some liberals and younger black voters thought she was not radical enough on the vital issues of police brutality and racism in the legal system.
Harris has taken a tougher stand since the killing of George Floyd, backing legislation to reform policing.
How much of a threat is Harris to Donald Trump?
The US President lost no time in attacking Kamala Harris, telling Fox News that she demonstrated her weakness as a candidate while running for the Democratic nomination.
Harris "said horrible things about Biden," Trump added, and "essentially called him a racist."
In reality, she said, “I do not believe you're a racist” before criticising Biden's past opposition to policies around school desegregation.
But the clash between Biden and Harris during a 2019 primary debate to which Trump was referring was a standout moment in the campaign and is likely to be exploited by the Republicans.
The president earlier told a White House briefing that Harris had been "extraordinarily nasty" to Supreme Court Judge Brett Kavanaugh during his confirmation hearings.
Yet the Republicans may have to find a more substantive line of attack against Joe Biden's new running mate.
In turn, Kamala Harris has not been afraid to brand Trump a racist, citing his comments on African countries and immigrants. "I don't think you can reach any other conclusion," she told The Root Institute, which campaigns for black issues, in 2019.
How is the Biden-Harris ticket likely to shape the election campaign?
The personal nature of Donald Trump's immediate attacks on Kamala Harris give an indication of what to expect between now and November.
Within minutes of the presumptive Democratic nominee’s announcement, false information was circulating on social media, claiming that Harris had called Biden a “racist” and that she is not eligible to run.
Harris, who was born in Oakland, California, has been the victim of online falsehoods for more than a year that say she is not eligible to become president because her parents were not born in America.
Some pro-Harris women's groups say they have been preparing a campaign of their own, aiming to shut down sexist coverage and disinformation as soon as it appears.
They say their efforts draw from the experiences of the 2016 campaign and the sexism Hillary Clinton faced from Donald Trump, some of his supporters and the media.
Christine Todd Whitman, a former New Jersey governor and Republican, said Biden's running mate will be at the centre of an “ugly” social media campaign from online bullies.
"This is going to be brutal because these platforms allow people to do things anonymously, saying things anonymously," Whitman said.