There is a reason as to why Biden was hesitant to leave the race and endorse Kamala Harris as the Democrat nominee: he was not very confident that she can beat Donald Trump, as we read in Axios:

President Biden hesitated to drop his re-election campaign in part because he and his senior advisers worried that Vice President Kamala Harris wasn’t up to taking on Donald Trump, according to three Biden aides familiar with recent talks about his plans.

One Democratic operative told Axios that most Democrats are disappointed with Kamala Harris:

One Democratic operative tells Axios’ Alayna Treene that most Democrats aren’t saying, “‘Oh, no, our heir apparent is f***ing up, what are we gonna do?’ It’s more that people think, ‘Oh, she’s f***ing up, maybe she shouldn’t be the heir apparent.'”

Numerous officials from Biden’s administration have described Harris’s operation as vice-president as a “shitshow”:

Several administration officials used “shitshow” when describing Harris’ office, and contrast her operation with disciplined, virtually leakproof Biden aides.

Some Biden officials view the Harris operation as poorly-managed and staffed with people who don’t have long-term relationships with her. They feel she’s gotten bad advice from her press and communications shop and think it’s telling that she’s already lost two advance aides and a digital director.