While I think there is a plausible path for President Biden to drop out of the race, the idea that Democrats could replace him with somebody other than Vice President Kamala Harris is pure fantasy.
Setting aside the obvious point about the optics in the modern Democratic Party of passing over somebody who would be the first black female president to plop in some white dude like California governor Gavin Newsom, there are many practical, logistical reasons why putting in Harris would be the most straightforward way for Democrats to proceed.
There isn’t much time to play with. Because of the early Ohio ballot deadline, the Democrats moved up their formal nomination of their presidential candidate to August 7, or fewer than six weeks from now. If Biden were to drop out of the race, Democrats need as clean of a solution as possible — a quick, rip-off-the-Band-Aid approach. The Harris option comes closest to that since she is already on the ticket and has served at his side for four years. He could simply release all his delegates, and urge them to support Harris. If that were the case, it could be coordinated so all of the top Democrats echo this sentiment, immediately issuing statements endorsing Harris. My guess is that outside of maybe a few gadfly types, Democrats would fall in line.
If Harris isn’t anointed, Democrats would need to open the process to anybody. If not Harris, why does it have to be Newsom? Why not Gretchen Whitmer? Or somebody else? This is a recipe for a Game of Thrones–style battle for the nomination that would have to be resolved without the input of voters, given that the primaries have all ended. Already faced with the prospect of changing horses four months out from the election, Democrats do not have six weeks to spare for a bloody fight.
Additionally, if Biden drops out, it would be an acknowledgment that Democrats cleared the field for a guy who it now turns out was not fit to serve another four years after all. If they sideline Harris, it would mean that they also nominated somebody for vice president who they do not believe is ready to step in if there is an emergency.
In addition to all of this, likely the biggest practical obstacle to choosing somebody else is that because Harris is already on the ticket, she would be able to tap into all the money they’ve raised for the campaign. Place in a new candidate, and Democrats would have to start raising money from scratch, hundreds of millions of dollars behind Donald Trump.
The extraordinary event of Biden dropping out of the race would not be an opportunity for Democrats to take a mulligan on their entire ticket. If it isn’t Biden, it is Harris or bust.