The 81-year-old dialled into a meeting from his home in Rehoboth Beach, Delaware, where he is recovering from COVID-19, to lend his support to Vice President Kamala Harris.
“It was the right thing to do,” he said.
He had planned to talk about his decision to step aside in an address to the nation later this week.
“The name has changed at the top of the ticket, but the mission hasn’t changed at all,” Biden said in his first public remarks since announcing his decision to step aside.
He promised he was “not going anywhere” and planned to campaign on Harris’ behalf.
As he handed off the mantle of leadership to Harris, Biden added: “I’m watching you kid. I love you.”
Worries over Biden’s fitness for office were replaced by fresh signs of unity after a seismic shift to the presidential contest that upended both major political parties’ carefully honed plans for the 2024 race.
Biden’s departure freed his delegates to vote for whomever they choose at next month’s convention.
Harris made her first public appearance since Biden dropped out of the race, paying tribute to the president’s legacy, which she described as “unmatched in modern history”.
Her brief comments came as the deluge of endorsements continued, including perhaps the most important early omission, Speaker Emerita Nancy Pelosi.
Speaking to campaign staff in Delaware, Harris acknowledged the “rollercoaster” of the last several weeks, but expressed confidence in her new campaign team.
“It is my intention to go out and earn this nomination and to win,” she said.
She promised to “unite our Democratic party, to unite our nation, and to win this election.”
The life of Joe Biden in pictures
She quickly leaned into the themes that will be prominent in her campaign against Trump over the coming 100 days, contrasting her time as a prosecutor with Trump’s felony convictions — “I know Donald Trump’s type,” she said — and casting herself as a defender of economic opportunity and abortion access.
“Our fight for the future is also a fight for freedoms,” she said.
“The baton is in our hands.”
More than 1,300 pledged delegates have told The Associated Press or announced that they plan to support Harris at the convention, more than half the 1,976 benchmark set by Democratic National Committee rules.
Winning the nomination is only the first item on a staggering political to-do list for her after Biden’s decision to exit the race.