Harris, 55, becomes the first black woman on a major presidential ticket in US history and providing him with a partner well suited to go on the attack against Republican President Donald Trump.

“I have the great honor to announce that I’ve picked @KamalaHarris — a fearless fighter for the little guy, and one of the country’s finest public servants — as my running mate,” Biden said on Twitter.

I have the great honor to announce that I’ve picked @KamalaHarris — a fearless fighter for the little guy, and one of the country’s finest public servants — as my running mate.

— Joe Biden (@JoeBiden) August 11, 2020

With social unrest over racial injustice and police brutality against black Americans rocking the country for months, Biden had been under increasing pressure to select a woman of colour as his running mate. She is also the first Asian-American on a major presidential ticket.

In Harris, a senator from California who made her own run for the White House before ending it and endorsing Biden, he gains a deeply experienced politician already battle-tested by the rigours of the 2020 presidential campaign as they head into the final stretch of the November 3 election.

Harris, who became only the Senate’s second black woman in its history when she was elected in 2016, will be relied on to help drive the African-American vote – the Democratic Party’s most loyal constituency. Four years ago, the first dip in black voter turnout in 20 years contributed to Democrat Hillary Clinton’s upset loss to Trump.

Biden served as vice president for eight years under president Barack Obama, the first black US president.

Harris, a former prosecutor and state attorney-general in California, is well known for her sometimes aggressive questioning style in the Senate, most notably of Brett Kavanaugh during his 2018 Supreme Court confirmation hearing.

The choice of a running mate has added significance for Biden, who will turn 78 in November and be the oldest person to become president if he is elected.

Biden’s age also has led to broad speculation he will serve only one term, making Harris a potential top contender for the nomination in 2024. Some of his allies were concerned that would make her a bad fit for the No.2 job and questioned her loyalty.

Biden publicly committed to choosing a woman as his No. 2 in a March debate after discussing the matter with his wife Jill and had considered other former presidential rivals such as Senators Amy Klobuchar and Elizabeth Warren.

After the protests that erupted over the May 25 killing of George Floyd, a black man, by a white policeman in Minneapolis, Biden’s search focused predominantly on candidates of colour.

Harris will be confirmed as Biden’s running mate at the Democratic convention that begins on Monday, where Biden will also be formally nominated to challenge Trump.

-AAP