JOE BIDEN chose Senator Kamala Harris as his running mate, Tuesday, betting that her ties to the African-American community and self-branding as a “progressive prosecutor” will help propel him to the White House.

Ms. Harris, 55, who ran against Mr. Biden in the Democratic presidential primaries, becomes the first Black woman and first Asian-American on a major party presidential ticket. Known as an aggressive campaigner, the junior senator from California has won statewide elections in the most populous US state three times. She built her early career as district attorney of San Francisco and later California attorney general.

“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,” Mr. Biden tweeted.

Ms. Harris tweeted her own response, pledging to help Biden defeat President Donald Trump.

“@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,” she said.

The Democratic nominee announced the selection six days ahead of the party’s convention, which begins Aug. 17.

The two candidates will appear together in a virtual fund-raiser on Wednesday and at events in Delaware in the coming week. Ms. Harris will face Vice-President Mike Pence in a Oct. 7 debate scheduled to be at the University of Utah.

Mr. Biden made his decision based on his own eight years as Barack Obama’s vice-president. He often said he hoped to choose a running mate with whom he’d be compatible and shares values, even if she came from a very different background than his.

Ms. Harris knew Mr. Biden’s son Beau when he was attorney general of Delaware and Ms. Harris held the same job in California. Mr. Biden was also impressed by her work on the Senate Judiciary Committee, the campaign said.

With her history in law enforcement, Ms. Harris could help Mr. Biden revamp the US criminal justice system, which has been under intense scrutiny since nationwide protests after the May 25 death of George Floyd in Minneapolis police custody. Her tough stance for police reform in recent months, including co-authoring a Senate bill to ban police chokeholds and other steps, has helped mute criticism from advocates of her own record as a prosecutor.

Ms. Harris has drawn criticism over her aggressive prosecution of men of color, some of whom were later exonerated, for serious crimes. Ms. Harris responded to that criticism by describing herself as a progressive prosecutor who worked to thread the needle between law-and-order toughness and a protective instinct for those who needed it.

Rashad Robinson, executive director of Color of Change, a political action committee, said Ms. Harris was a “stellar Black candidate” even if they haven’t always agreed on issues.

But Mr. Robinson said Ms. Harris needs to directly confront her past as a prosecutor and talk about how her views have evolved.

“These are questions that she has to answer head on, she actually has to talk about her evolution and talk about the way the world was in terms of her progressive prosecution back when she was a prosecutor, talk about the challenges that she’s had and talk about where she’s moved,” Mr. Robinson said in a phone call with reporters.

Mr. Biden committed to choosing a woman even before he’d won the nomination, making a surprise announcement during the final debate of the primary in mid-March. Ms. Harris endorsed Mr. Biden on March 8, throwing her support behind his candidacy after sharply criticizing him in the Democratic primaries.

Representative Jim Clyburn, whose support gave Mr. Biden the momentum he needed to win South Carolina and eventually the nomination, welcomed the historic choice.

“What we’re seeing today is one more chapter in this country living out its true creed that all men and women are created equal,” he said.

Mr. Obama said Mr. Biden had “nailed this decision.”

“Now Joe has an ideal partner to help him tackle the very real challenges America faces right now and in the years ahead,” Mr. Obama said in a statement.

Ms. Harris and Mr. Biden had a heated exchange during the primary, when she confronted him over his opposition in the 1970s to a federal mandate for busing to integrate schools, which put him on the same side as segregationist senators. Mr. Biden has said he supported desegregating schools, but not the federal mandate.

After the debate, it became clear she and Mr. Biden agreed on modern desegregation policies.

Mr. Trump said he was “a little surprised” that Mr. Biden selected Ms. Harris as his running mate, saying she had been “nasty” to the former vice-president in primary debates, using a term he usually reserves for women. He criticized her for her aggressive questioning of Supreme Court Justice Brett Kavanaugh over sexual misconduct allegations during his Senate confirmation hearings.

“I thought she was the meanest, the most horrible, the most disrespectful of anybody in the US Senate,” Mr. Trump said. — Bloomberg