Joe Biden is surely not going to want to put himself forward as candidate for 2024. He is heading for his 80s and that is showing and indeed it might not be that he is able to complete his first term.
If Biden steps down before 2024 then Harris will become president and surely she would be the nominee in the forthcoming election. It’s hard to see anything else.
My guess is that Biden will seek to carry on in the post until his first term is finally over which makes it harder for Harris. He will need to make an announcement about his intentions sometime before the election and the best moment, surely, will be in a year’s time after the Midterms.
That would trigger a nomination race and the two leading contenders would be the vice president and the current transportation secretary. Given what we know from last time Buttigieg would be a much better contender if it gets to that stage. Harris pulled out of the the 2020 race even before the first primary.
Buttigieg competed in the first few contests and indeed was the victor in the Iowa caucuses.
I’m on Buttigieg at 66/1 for the Presidency.