Rick H. gets the heart of the issues right in his early post. I want to highlight some more wrinkles (but I put the odds of Biden stepping down much lower than 50%).

First, Rick is right that the DNC rules for “pledged” candidates really just a pledge and not binding. Per IX.E.3.d, “All delegates to the National Convention pledged to a presidential candidate shall in all good conscience reflect the sentiments of those who elected them.” Likewise, IX.C.7.e, “Eligible delegates may vote for the candidate of their choice whether or not the name of such candidate was placed in nomination.” (Rick rightly notes, “That would be true even if Biden stayed in the race,” but this is not a politically likely option.)

Second, in the event of a vacancy in the ticket after the convention, the rules are a little different: “Filling a Vacancy on the National Ticket: In the event of death, resignation or disability of a nominee of the Party for President or Vice President after the adjournment of the
National Convention, the National Chairperson of the Democratic National Committee shall confer with the Democratic leadership of the United States Congress and the Democratic Governors Association and shall report to the Democratic National Committee, which is authorized to fill the vacancy or vacancies.”

Third, the DNC announced it would hold a “virtual roll call” before the convention. That was when there was some doubt about Ohio’s ability to amend its ballot access rules. But perhaps more interestingly, even though Ohio has amended its law, it appears the DNC might worry that other deadlines in other states might be a problem, or in states where a “provisional certification” previously was sufficient for a presumptive nominee that might be legally challenged in this election. Regardless, it’s not clear how this would work in the event of a contested convention, and the DNC might have to backtrack if that’s the case.

Fourth, “superdelegates” (or “automatic delegates”) are eligible only on the second ballot in the event no candidate has a pledged majority the first time around (IX.C.7.b). So the ground could shift from the first to the second ballot separate and apart from any jockeying after the first ballot and candidacies.

Fifth, and finally, recall that New Hampshire violated the DNC’s rules by going early with its primary. In eras of consensus (think to the 2008 Michigan and Florida fiasco, resolved only once Barack Obama secured a majority of the delegates and those states’ rule-breaking delegations could be seated), there is little dispute when some states violate party rules. But in a contested convention, the decision whether to seat a batch of rule-breaking delegates will be much more fraught.