In the 2008 campaign she embraced Mr. Biden’s ultimately doomed presidential run and then his selection as Barack Obama’s running mate, and relished her work as second lady. But in 2015, devastated by the death of their elder son, Beau, the Bidens passed on another race.

This time, according to interviews with more than 20 friends, former staff members and Democratic officials, Dr. Biden is fully committed to her 77-year-old husband’s candidacy, serving privately as a trusted confidante who can offer unvarnished feedback in a way few others can, and publicly as an active surrogate. She adheres to her stump speech more than her sometimes-meandering husband does to his, and while she does not speak in smooth sound bites, after her tour as second lady she is more comfortable onstage.

“She’s like the adviser in chief,” Mr. Kaufman said. “Kind of like the relationship he had with Obama, he’d be the last person in the room — that’s Jill.”

Dr. Biden’s campaign aides declined repeated requests to make her available for an interview.

Dr. Biden, 68, whose lingering accent and sports loyalties reflect her Philadelphia-area childhood, has a reputation for being direct and energetic, the type to call former staff members to sing “Happy Birthday” and to enjoy practical jokes — she concluded a recent campaign video by smearing pie on her press secretary’s face (“caucusing for Joe” is “easy as pie”). She is not a political obsessive — which can lead, occasionally, to off-message moments.

In August, for instance, she advised Biden skeptics they should vote for her husband even if they were not enthusiastic about him, saying, “Maybe you have to swallow a little bit and say, ‘O.K., I sort of personally like so-and-so better,’ but your bottom line has to be that we have to beat Trump.” And she made headlines in May for saying that “it’s time to move on” from controversy surrounding her husband’s treatment of Anita Hill during Justice Clarence Thomas’s 1991 Supreme Court confirmation hearings.