A former Obama strategist told The Telegraph that they favoured a swift hand-over without opening the floor to competition against Ms Harris.
“We’re 100-ish days from the election. I don’t think we have time for a process,” the strategist said, adding: “I think the vice-president is our only choice right now.”
The strategist, a veteran of numerous presidential campaigns, said Ms Harris was an “upgrade” on Mr Biden “in every way”.
“There’s not a single attack on Donald Trump that Joe Biden can make that Kamala Harris can’t make even better,” the strategist said.
The Obama-era veteran said Ms Harris could use her background as a former district attorney to her benefit. “This is somebody who convicted a lot of felons, running against a convicted felon. I’ll take that,” they said.
Another Democrat insider said, despite reservations about Ms Harris’s performance as vice-president, she has “grown” into the role.
The insider noted that as vice-president, Ms Harris has a rare insight into the challenges facing the US through her daily intelligence briefings.
“I think that has made her a deeper thinker,” he said, describing her having “an understanding of the world that she didn’t have three and a half years ago”.
Shifting polls
Polls suggest the public perception of Ms Harris has shifted following Mr Biden’s abysmal debate performance.
While Trump leads Mr Biden in most current polls, a recent survey by CNN had Ms Harris tying with the Republican nominee.