There, a candidate must win support from the majority of “delegates” - party officials who formally choose the nominee. Delegates are assigned to candidates proportionally based on the results of each state’s primary election. This year, Mr Biden won almost 99% of the nearly 4,000 delegates.
According to the DNC rules, those delegates are “pledged” to him, and are bound to support his nomination.
But if Mr Biden were to drop out, it would be a free-for-all. There is no official mechanism for him or anyone else in the party to choose his successor, meaning Democrats would be left with an open convention.
Presumably, Mr Biden would have some sway over his pledged delegates, but they would ultimately be free to do as they please.
That could lead to a frantic contest erupting among Democrats who want a shot at the nomination. Source