Biden said in a tweet he would deliver a prime-time address to the nation on Wednesday evening to explain his decision to withdraw, and lay out a vision for his final six months in the White House.
They included the governors Roy Cooper of North Carolina, Tim Walz of Minnesota, and Gretchen Whitmer of Michigan, although Whitmer has said she would serve instead as co-chair of the campaign.
A survey by the Associated Press indicated that Harris had the backing of 2,688 state delegates, far more than the 1,976 needed to become the nominee.
A CBS News/YouGov poll on Tuesday found that 83% of Democratic registered voters approved of Biden withdrawing from the race, while just 17% disapproved.
Harris entered the second full day of her campaign for the nomination in an almost unassailable position, following a breathless 24 hours that saw almost every senior party figure championing her candidacy.
All 23 Democratic state governors have publicly backed Harris, including several who had been considered potential rivals for the nomination, such as Whitmer and JB Pritzker of Illinois.
The rapid pace at which she racked up endorsements was matched by an avalanche of donations. More than $100m poured into campaign coffers in its first day, a spokesperson said on Monday, calling it the largest single-day haul of any presidential candidate in history and with most of the money coming from grassroots donors making their first contributions of the election cycle.
A number of organizing efforts are also under way on her behalf. The Win With Black Women advocacy group hosted a Zoom call for 44,000 people and raised more than $1.5m. A similar initiative involving more than 20,000 Black men on Monday pulled in at least another million.
Among the high-profile endorsements to come in for Harris on Tuesday was that of George Clooney, the Hollywood actor who wrote a powerful opinion article earlier this month calling for Biden to step aside.
