That group, working in pairs, quickly cast a wide net of some 20 women whom they thought should be considered, multiple individuals involved with the process told ABC News.
But that list winnowed to just 11, which involved a more invasive round of questioning by search committee members and teams of lawyers under the strictest agreements of confidentiality.
Eventually the pool would narrow further to only the women left in final consideration, with Sen. Kamala Harris and former National Security Adviser Susan Rice nearing the top of the list.
Biden's selection of Harris, the first Black woman, the first woman of Indian descent and only the fourth woman in the country’s history to be chosen to join a major party ticket, marks another historic bid for Biden.
While Harris was always a favorite, her selection was not a foregone conclusion. The decision to ask her to join the ticket was the result of three months of work that put candidates through hours-long formal interviews and follow up calls from teams of lawyers, as well as the evaluation of a massive volume of documents submitted by the women themselves — a tedious task meant to leave no stone unturned.
In reconstructing the search that led to Biden’s decision, ABC News spoke with multiple people familiar with the process and close to the women under consideration.
Biden's committee of allies and close advisers, who were charged with overseeing the meticulous and secretive process, included: former Connecticut Sen. Chris Dodd, Delaware Rep. Lisa Blunt Rochester, Los Angeles Mayor Eric Garcetti and Cynthia Hogan, a longtime Biden aide.
After an initial round of interviews with nearly two dozen women, the team presented their findings to Biden to pare down the list for more in-depth assessments.
The core group of 11 candidates underwent what was described as an "invasive," "time consuming" and "extensive" vetting, which involved a lengthy questionnaire for the women of more than 120 questions. The inquiries ranged from resume details to biographical ones and also explored any potential legal, medical or political liabilities that could be leveled against the candidates.
Questions fell along the lines of 'is there anyone who you think fairly or unfairly would attack your appointment?' or 'what’s a political stand that you’ve taken that you’re particularly proud of?'
High-profile attorneys, including former White House counsel Bob Bauer, campaign general counsel Dana Remus and former Homeland Security Adviser Lisa Monaco, along with teams of lawyers, managed the thorough legal vetting of the nearly dozen women.
The attorneys spoke with some of the candidates for up to two and a half hours and conducted multiple follow-up calls for additional questions and clarifications, and candidates were also asked to compile extensive public, private and political documents and records for review — an arduous undertaking for the candidates and their aides.
The co-chairs worked in earnest in pairs — Blunt Rochester and Dodd, and Garcetti and Hogan — to run through the exhaustive process, interviewing a trimmed-down list of candidates more than once before Biden made his decision.
A second round of interviews with the vetting committee in part sought to focus on each of the candidates’ governing philosophies on issues that were central to the Democratic primary, such as health care. The interviews also hearkened back to some of the key dynamics that played out during the 2020 primary race, including questions around whether the women considered themselves more of a compromiser or an ideological purist.
In total, the vetting committee racked up 120 hours worth of interviews throughout the more than three-month deep-dive, and against the backdrop of some of the most unprecedented circumstances for the country.
The coronavirus transformed much of how the search was run, and the protests in the wake of the death of George Floyd subjected Biden to significant public pressure to choose a Black woman.
The extraordinary crises helped to raise some of the contenders' national profiles and possibly boost their prospects throughout their auditions, while also challenging them amid the daunting scrutiny that comes with the vetting process.
Michigan Gov. Gretchen Whitmer, one of the women who garnered national prominence for her early and aggressive response to the coronavirus pandemic, and Sen. Tammy Duckworth, D-Ill., who was drawn into the spotlight by President Donald Trump’s use of military force during the demonstrations over race, met with Biden in the two weeks before he made his choice.
Both Whitmer and Duckworth, who weren’t initially considered to be among the top competitors, emerged as formidable contenders as the process extended into the summer months.
Harris, who was long considered the frontrunner, was meeting with Biden as early as July.
If Biden had a preferred pick, he never showed publicly an inkling of who that might be, but sources close to the vetting committee felt that the parallel race for the vice presidency was always Harris’ to lose.
The vetting committee often spoke with Biden throughout the process, but no formal recommendation was made to him.
Following the extensive vetting, the presumptive Democratic nominee conducted one-on-one interviews with his short list of women, both in-person and virtually.
Biden had long been clear about what he was looking for in a No. 2 — one that he would have personal chemistry with, as he did with former President Barack Obama, or as he said, would be "simpatico" with him in governing. He also wanted a partner who was prepared to step into the role on "day one" — a reflection of the reality of Biden’s age, as he would be the oldest president inaugurated at 78 if he were elected in November.
But the most salient factor that loomed over both Biden’s entry into the race and his final decision for a running mate was his son, Beau.
"I first met Kamala through my son Beau," Biden wrote in an email to supporters announcing his decision. "There is no one’s opinion I valued more than Beau’s and I’m proud to have Kamala standing with me on this campaign."
The Sunday night before Biden made his decision public, the vetting committee — having wrapped up their work — were simply awaiting the announcement, the final step in the process that fell only to Biden.
Just hours before Biden’s history-defining decision was made public, he spoke with some of the women he considered but ultimately passed on, multiple sources tell ABC News.
In his conversion with Duckworth, a source said, Biden thanked her for participating in the process and informed her she was not the one he was asking to join him on the ticket.
After Rep. Karen Bass spoke with Biden on Tuesday, the chair of the Congressional Black Caucus went ahead with a Zoom roundtable about a bipartisan bill on health standards for pregnant female inmates, even as her phone was abuzz with activity about Biden's announcement, a source familiar with the call told ABC News.
The process itself elicited mixed feelings as some of those in contention felt, at times, entirely in the dark about where they stood as the search wore on and others felt it was professional and thought Biden to be kind and gentlemanly throughout, a source close to one of the women, who was informed personally from Biden that she was not the pick, said.
In the end, Biden’s decision to choose Harris reflected what he often said publicly: He wanted a partner that he knew.
The search culminated in a single conversation on Tuesday afternoon. Biden asked Harris one simple question: "You ready to go to work?"
She first paused, before replying, "Oh my god. I am so ready to go to work," over a Zoom call.
For the other woman who were not picked to serve in the second-highest office in the country, the prospect of working in a Biden administration appears to remain on the table should Biden emerge victorious in the fall.
In one of his phone calls on Tuesday, a source with knowledge of the conversation said Biden raised the prospect of the candidate, who was not selected for the vice presidential post, serving in another role in his cabinet.
"The task before the administration will be huge. There’s many tasks, and I'll need a partner to help me tackle them. But it doesn't stop at the top," Biden said in July.
ABC News’ Zohreen Shah, Benjamin Siegel and Averi Harper contributed reporting.