Her arrival at Mr. Biden’s Philadelphia headquarters could assuage a number of Democrats, including Mr. Obama and his acolytes, who have long been uneasy about the former vice president’s operation. In addition to her own connection to the president, Ms. O’Malley Dillon’s husband, Patrick Dillon, also worked in the Obama White House.

Ms. O’Malley Dillon’s hiring was immediately greeted with enthusiasm by a number of Democrats. “I will no doubt be one of a chorus to say this, but absolutely excellent move by Joe Biden,” said Todd Schulte, a progressive advocacy leader and campaign veteran.

Mr. Biden already shook up his campaign last month, installing Anita Dunn, a longtime adviser, as his chief strategist. She is expected to remain in that role.

Greg Schultz, the original campaign manager, clashed with some in Mr. Biden’s orbit but is well-liked by a number of colleagues. Some of his admirers pointed out that, for all the criticism the campaign has received, Mr. Biden is now on the doorstep of the Democratic nomination after the campaign played out as Mr. Schultz had predicted.

For months, he assured Democrats that even if Mr. Biden were to struggle in heavily white Iowa and New Hampshire, which voted first this year, he would recover in more diverse Nevada and South Carolina and be catapulted into Super Tuesday.

Mr. Schultz was planning to stay on the campaign to work on “organizational planning for the general election,” according to Mr. Biden’s statement.

That task, said Robby Mook, the campaign manager for Hillary Clinton in 2016, “is an enormous undertaking” and suits Mr. Schultz, who is close to a number of national and state party officials. “Greg comes with the deep trust of the vice president and long experience building state operations,” Mr. Mook said.