Mr. Biden’s age — 77 — also may have heightened the importance of finding a running mate with thoroughly convincing political credentials. Mr. Biden himself seemed sensitive to that reality, reiterating often that he wanted a vice president who would be ready to assume the top job immediately.
“The first and most important attribute is, if something happens to me, the moment after it does, that that person is capable of taking over as President of the United States of America,” he said at a fund-raiser in May.
The vice-presidential search was at once highly public — involving tryouts on television and in online campaign events for more than half a dozen candidates — and surprisingly discreet for a campaign that has weathered a sizable number of leaks over the last 15 months.
Much of the process was carried out by a committee of four trusted advisers named by Mr. Biden in late April: former Senator Christopher J. Dodd of Connecticut, Representative Lisa Blunt Rochester of Delaware, Mayor Eric Garcetti of Los Angeles and Cynthia C. Hogan, Mr. Biden’s former chief counsel.
Aided by a team of lawyers, this group held interviews with a range of vice-presidential prospects and delved into their political records, personal finances and private lives before referring a smaller number of them for interviews with Mr. Biden.
The field of women considered was certainly the most diverse array of vice-presidential candidates in history, beginning with a pool of more than a dozen contenders that included governors, senators, members of the House, a former United Nations ambassador, the mayor of Atlanta and a decorated combat veteran. The group included two Asian-American women and the first openly gay person elected to the Senate. Mr. Dodd in particular is said to have pressed for a large list with some unconventional names on it, to give Mr. Biden maximum flexibility in his choice.
By the end of June, a smaller cluster of candidates had emerged as strong contenders, impressing the screening committee in interviews and reaching a point in the process that involved extensive document requests from Mr. Biden’s lawyers. Among that group were Ms. Harris, Senators Elizabeth Warren of Massachusetts and Tammy Duckworth of Illinois, Representative Val Demings of Florida, Gov. Michelle Lujan Grisham of New Mexico, Ms. Bass of California and Ms. Rice, the former national security adviser and United Nations ambassador.