It’s a bit complicated. Biden has more than enough electors (they are required to vote for him) to win in the first round of voting. If he steps aside before the convention, all his electors are free to vote for any candidate. That means an ugly convention fight. Also, it wipes Harris off the ballot. She does not automatically become the nominee or stay VP.
If Joe becomes the nominee then steps down, the DNC leaders pick whoever they want, after some input from governors and top congressional leaders.
Again, Harris disappears.
Expect the second scenario to occur. It has less negativity for the DNC. They do have to declare the nominee no less than 60 days before the election.
My guess is Joe will have an accident after the convention. Maybe he will have to take the big stairs on AF1.
