It was early in Joseph R. Biden Jr.’s vice-presidential search when he asked his advisers a sensitive question about Senator Kamala Harris. He kept hearing so much private criticism of her from other California Democrats, he wanted to know: Is she simply unpopular in her home state?

Advisers assured Mr. Biden that was not the case: Ms. Harris had her share of Democratic rivals and detractors in the factional world of California politics, but among regular voters her standing was solid.

Mr. Biden’s query, and the quiet attacks that prompted it, helped begin a delicate audition for Ms. Harris that has never before been revealed in depth. She faced daunting obstacles, including an array of strong competitors, unease about her within the Biden family and bitter feuds from California and the 2020 primary season that exploded anew.

Though Ms. Harris was seen from the start as a front-runner, Mr. Biden did not begin the process with a favorite in mind, and he settled on Ms. Harris only after an exhaustive review that forged new political alliances, deepened existing rivalries and further elevated a cohort of women as leaders in their party.

Ms. Harris was one of four finalists for the job, along with Senator Elizabeth Warren of Massachusetts, Gov. Gretchen Whitmer of Michigan and Susan E. Rice, the former national security adviser. But in the eyes of Mr. Biden and his advisers, Ms. Harris alone covered every one of their essential political needs.

Ms. Rice had sterling foreign-policy credentials and a history of working with Mr. Biden, but was inexperienced as a candidate. Ms. Warren had an enthusiastic following and became a trusted adviser to Mr. Biden on economic matters, but she represented neither generational nor racial diversity. Ms. Whitmer, a moderate, appealed to Mr. Biden’s political and ideological instincts, but selecting her also would have yielded an all-white ticket.

Other candidates rose and faded in the process: Senator Tammy Duckworth of Illinois powerfully impressed Mr. Biden’s search team, but his lawyers feared she would face challenges to her eligibility because of the circumstances of her birth overseas. Representative Karen Bass of California emerged as a favorite among elected officials and progressives — Speaker Nancy Pelosi spoke glowingly of her to Mr. Biden — but the relationship-focused Mr. Biden barely knew her.

In the end, Mr. Biden embraced Ms. Harris as a partner for reasons that were both pragmatic and personal — a sign of how the former vice president, who is oriented toward seeking consensus and building broad coalitions, might be expected to govern. Indeed, Mr. Biden has already told allies he hopes a number of the other vice-presidential contenders will join his administration in other roles.

This account of Mr. Biden’s decision is based on interviews with more than three dozen people involved in the process, including advisers to Mr. Biden and Ms. Harris, allies of other vice-presidential prospects and Democratic leaders deeply invested in the outcome of the search.

Mr. Biden’s instincts were not destined to lead him to Ms. Harris: He and members of his family had long expressed discomfort with the way she attacked him at a Democratic primary debate, and his political advisers remembered well the seemingly constant dysfunction of her presidential campaign.

There was a particular distrust in the Biden camp for the sharp-elbowed California operatives with whom Ms. Harris has long surrounded herself, fearing that they might seek to undermine Mr. Biden in office to clear the way for Ms. Harris in 2024.

Yet no other candidate scored as highly with Mr. Biden’s selection committee on so many of their core criteria for choosing a running mate, including her ability to help Mr. Biden win in November, her strength as a debater, her qualifications for governing and the racial diversity she would bring to the ticket. No other candidate seemed to match the political moment better.

ImageJoseph R. Biden Jr. embraced Ms. Harris as a partner for reasons that were both pragmatic and personal.
Credit…Erin Schaff/The New York Times

Harry Reid, the former Senate majority leader, said race had been essential to Mr. Biden’s decision.

“I think he came to the conclusion that he should pick a Black woman,” Mr. Reid said. “They are our most loyal voters and I think that the Black women of America deserved a Black vice-presidential candidate.”

Ms. Harris worked to soothe misgivings in the Biden family, including from Jill Biden and Valerie Biden Owens, Mr. Biden’s sister and longtime adviser. But Ms. Harris also drew upon a family link unmatched by any other candidate: her friendship with Mr. Biden’s elder son, Beau, who died from cancer in 2015.

The potential for conflict between Biden and Harris advisers was resolved in another way, at least for now: Mr. Biden and his advisers conveyed to Ms. Harris that they expected to have the same understanding with respect to staff hiring that Mr. Biden had followed with former President Barack Obama. During the campaign and, if they win, during a Biden-Harris administration, Ms. Harris’s staff hiring would be approved by Mr. Biden.

Jennifer O’Malley Dillon, Mr. Biden’s campaign manager, told Ms. Harris plainly after she was picked that they would be one team, and that she would have the full support of the Biden staff. In a statement Thursday night, after this story was published online, Ms. O’Malley Dillon said some Harris aides would be coming on board.

“We’ve already begun welcoming members of Senator Harris’ team to the campaign and are all moving forward together, as one unit focused on beating Donald Trump this fall,” Ms. O’Malley Dillon said.

But other Biden advisers made clear that selecting Ms. Harris for the vice presidency did not mean selecting her full political entourage for jobs in the campaign or government — a reality Ms. Harris is said to have accepted.

Within the Biden team, it was understood that rule would apply even to her sister, Maya Harris, a former Hillary Clinton adviser who is Kamala Harris’s closest confidante. But a Biden spokesman said on Thursday night that the matter of Maya Harris did not come up in conversations with the senator.

Searching for a Partner

Having been through a vice-presidential search himself, Mr. Biden was clear from the start about what he wanted in a running mate — and in a selection process. He wanted a full partner in government with whom he felt personally “simpatico.” He did not want a “Survivor”-style process of elimination whereby a large pool of candidates would be gradually slashed down, with the losers identified as such in public, according to people who spoke to him about the process.

And for the most part, that is what Mr. Biden got — a discreet search team, led by four Democratic dignitaries, that held interviews with about a dozen women, a smaller number of whom were then asked to turn over a huge volume of private documents for review. To ensure the contenders’ privacy, he did not allow even his senior staff members to see some of their most personal vetting information.

Mayor Eric Garcetti of Los Angeles, one of the members of the search team, said Mr. Biden had been emphatic that the process should unfold in a dignified manner that would leave all the participants better off.

“He was committed to this being a career elevation for everybody, and finding the right running mate, and he did both,” said Mr. Garcetti, who declined to comment on the details of the search.

The interviews conducted by Mr. Biden’s search team were revealing and, in some cases, surprising — not because of confidential and damaging information that came to light, but because of the personal candor and raw political ability that some candidates brought to the conversations.

Two of the standout interviews were with Ms. Duckworth, an Asian-American veteran of the Iraq war, and Gov. Gina Raimondo of Rhode Island, a centrist with formidable academic and business credentials. Both left the search committee dazzled, but they faced other obstacles — in Ms. Raimondo’s case, her limited national profile and adversarial relationship with influential labor unions.


Credit…Erin Schaff for The New York Times

Ms. Duckworth was regarded by Biden advisers as among the candidates likeliest to help him achieve a smashing electoral victory in November. But legal advisers to the campaign expressed urgent concern that Ms. Duckworth could face challenges to her nomination in court: She was born overseas, to an American father and a Thai mother. While Mr. Biden’s team believed Ms. Duckworth was eligible for national office, campaign lawyers feared that it would take just one partisan judge in one swing state to throw the whole Democratic ticket off the ballot.

Ms. Warren, too, was persuasive and compelling to the search committee in her interviews, pleasantly surprising a largely moderate panel, including several members who had looked askance at some of the policies and language she adopted in her own presidential campaign. But Ms. Warren told the committee she fully appreciated that the role of the vice president was different, and that the agenda of a Biden administration would be Mr. Biden’s.

“He won; I lost,” Ms. Warren said in one interview, according to people briefed on her comments.

What’s more, Ms. Warren noted that she was past her 70th birthday, and would not be looking to advance a long-range political career in the vice presidency, leaving some members of the search team convinced she did not aim to run for president again. The search team told Mr. Biden they believed they could rely on Ms. Warren as a cooperative governing partner — an assessment Mr. Biden shared.


Credit…Ruth Fremson/The New York Times

Of all the interviews conducted, only Ms. Harris’s burst into public view as a matter of controversy, when one of the members of the search team, former Senator Christopher J. Dodd of Connecticut, told associates that he had been dismayed by what he regarded as an inadequately contrite answer by Ms. Harris about her searing denunciation of Mr. Biden at a Democratic primary debate in June 2019.

Ms. Harris recognized from the start that her attack on Mr. Biden — for having worked with segregationist senators to oppose school busing — was a liability for her as a potential running mate, and she spent considerable time reaching out to Biden allies to seek their advice about how she should approach the former vice president.

One longtime Biden supporter told her bluntly that she should make clear she would not upstage Mr. Biden in the campaign, telling her, “You don’t need to be Sarah Palin to his John McCain.”

Another Biden ally, who served with him in the Obama administration, urged Ms. Harris to at least implicitly engage on the topic of their debate clash, proposing that she bring up George H.W. Bush’s criticism of Ronald Reagan’s “voodoo economics” in the 1980 Republican primary — an attack that did not stop the two from serving beside each other for eight years.

But Ms. Harris’s interviews covered far more ground than just a single debate, and like the other candidates, Ms. Harris faced intensive scrutiny of her personal and political history. Biden advisers asked, for instance, about contributions she received as state attorney general from Steven Mnuchin, President Trump’s Treasury secretary, who at the time was running a bank, OneWest, that was accused of violating foreclosure laws. Ms. Harris declined to pursue prosecutions in the case.

Ms. Harris has said consistently that political donations played no role in her legal decisions as attorney general.

In her interviews, and in a final-round conversation with Mr. Biden, Ms. Harris was emphatic on one point: that she would be loyal to Mr. Biden and support his agenda without reservation, according to a Biden aide briefed on their discussion.

Deliberation and Debate

By July, Mr. Biden and his team were converging on a theory of his decision, if not yet an actual vice-presidential pick.

There was broad agreement among his advisers that Mr. Biden should choose a woman of color, though Mr. Biden remained drawn to both Ms. Whitmer and Ms. Warren. There was unanimity that he needed someone with unimpeachable governing qualifications: Private Democratic polling and focus groups found that voters were keenly aware of Mr. Biden’s advanced age, and the possibility that his running mate could become president by medical rather than electoral means.

In some Democratic focus groups, too, voters expressed skepticism that Biden would choose a candidate with strong qualifications: By making gender a nonnegotiable requirement, they wondered, was Mr. Biden indicating he cared more about identity than experience? To Democratic strategists who have studied the obstacles for women in politics, the presumption that there would be better credentialed men available was not a surprising concern.

At least two women besides Ms. Harris seemed capable of matching all those criteria: Ms. Rice and Ms. Bass, the former speaker of the California Assembly.


Credit…Doug Mills/ The New York Times

Ms. Rice benefited from her close relationship with Mr. Biden and a concerted push on her behalf by other alumni of the Obama administration, though not the former president himself. But she had never been a candidate for office before, and Mr. Biden was more familiar than most with how much of a vice president’s time is typically spent on political errands. He concluded it would be too risky to pick a running mate who had never been on the ballot.

Ms. Bass emerged late in the process as a formidable rival to Ms. Harris. Though she was little known outside California and Congress, Ms. Bass impressed the vetting committee, and Mr. Dodd took steps to elevate her during the search process. Several people close to Mr. Biden sang her praises to the former vice president, including Ms. Pelosi and Senator Chris Coons of Delaware.

But Ms. Bass knew she had political liabilities, according to people who spoke with her directly throughout the process. She had visited Cuba repeatedly as a young woman and at times had made somewhat admiring comments about the government of Fidel Castro. She discussed those matters openly with the vetting committee, recognizing how politically damaging they could be in the crucial swing state of Florida, with its large and politically active immigrant communities from repressive Latin American countries.

Mr. Biden was aware of Ms. Bass’s Castro-era baggage well before it spilled into the news media. He told one longtime friend that her history with Cuba could cause political headaches, though to other people he suggested he did not see it as politically disqualifying — he intended to win the election in the Midwest, Mr. Biden told them, even if he were to fall short in Florida.

For Mr. Biden, Ms. Bass’s greatest shortcoming as a candidate was simpler: He did not really know her, and the coronavirus pandemic made it difficult to establish a close personal connection in short order.

One candidate who did forge such a bond with both Joe and Jill Biden was Representative Val Demings of Florida, a former Orlando police chief whom one adviser said the Bidens “loved.” Ms. Demings’s background in law enforcement may have hindered her in the vice-presidential search — Mr. Biden was briefed on specific allegations of police misconduct on her watch — but some Biden advisers are hopeful she will challenge Senator Marco Rubio in the 2022 election.

As Mr. Biden’s private deliberations wore on, the public dimension to the process began to grow ugly. A report in Politico on Mr. Dodd’s criticism of Ms. Harris enraged her admirers, and this week some of Mr. Biden’s top aides, still irritated at Mr. Dodd’s apparent lapse in discretion, sought to downplay the selection committee’s clout, suggesting its members had no more pull than his other advisers.


Credit…Anna Moneymaker for The New York Times

Supporters of Ms. Harris saw the late surge of advocacy for Ms. Bass — another, more liberal Black woman from California — as the equivalent of a torpedo aimed at Ms. Harris alone, while allies of Ms. Bass and Ms. Rice privately complained that they believed Ms. Harris’s political advisers were circulating negative information about them to the news media.

Mr. Biden and his top aides were cognizant of the sniping, but advisers stressed to the former vice president that there was no way of knowing if it was authorized by Ms. Harris or was being done on a freelance basis — and that they shouldn’t let it color their decision.

Some Democratic women were uneasy, though, about how much criticism all four finalists faced, and made little attempt to hide their frustration.

“We need to be celebrating these women,” said Representative Debbie Dingell of Michigan. “They are all talented, passionate, capable people.”

Mr. Biden’s mind was nearly made up by the end of the weekend, but he kept talking with advisers into Monday. On Tuesday morning, the campaign set in motion the announcement that became public within hours. And Mr. Biden went about the hard business of letting down the runners-up that he had come to value as allies and friends.

One by one, Mr. Biden told them he hoped to have them “on the team” in one way or another, according to people briefed on his calls.

To Ms. Harris, he placed a video call and asked, “You ready to go to work?”