Democratic US presidential candidate Joe Biden has chosen Kamala Harris as his running mate for the November 3 election.

The California senator has become the first black and South Asian American woman to serve in the role, which will mean she becomes vice-president if Mr Biden beats Donald Trump.

He made the announcement on Twitter on Wednesday morning (Australian time), saying “I have the great honour to announce that I’ve picked Kamala Harris”.

He described Ms Harris, 55 – who was initially his opponent but dropped out of the race in December – as “a fearless fighter for the little guy, and one of the country’s finest public servants”.

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The selection comes months after Mr Biden, who was Barack Obama’s vice-president, pledged to elevate a woman as his running mate.

Ms Harris had worked closely with his late son, Beau, when she was California’s attorney-general, Mr Biden said.

“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.”

Tweet from @JoeBiden

It is the first time a major US party has put a black woman on the presidential ticket.

Mr Biden has faced increasing pressure to tap a woman of colour following months of protests about racial inequity after the police killing of George Floyd, a black man, in Minneapolis.

Mr Biden, 77, is leading Mr Trump, 74, in national opinion polls.

Less than an hour after Mr Biden’s announcement, Mr Trump released an attack video calling Ms Harris a “phoney”.

“I thought she was the meanest, most horrible, most disrespectful person in the whole senate,” he said.

Ms Harris, who became only the second black female US senator 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 Mr Trump.

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

She’s also not shied away from criticising Mr Biden.

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

Mr Biden’s age also has led to broad speculation he will serve only one term, making Ms 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.

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

-with AAP