Slowly rolling out significant endorsements so that VP Harris is always in the news may be what Obama and Pelosi are doing. From what I've seen/heard on Obama/Pelosi delay it may be that they are showing respect for Biden and his accomplishments by holding off some before endorsing. Another take I've heard is that Obama/Pelosi want to make sure Harris is the nominee before endorsing her and then low and behold someone else is nominated instead. I'm not sure if they are supporting basically a floor fight at the convention, albeit a lopsided one. I think most, if not all, of the state Democratic parties have put their support behind VP Harris. State convention delegates are meeting to see if they are throwing their support behind Kamala or not. 

There is going to be a virtual nomination vote by all of the delegates the first week of August so that the Democratic nominee can be on the ticket in Ohio. That means that BEFORE the actual DNC starts, the nominee would have already been confirmed as the official nominee. I’m sure there could be some people trying to put someone else on the ticket but that would be known and resolved one way or the other in just a few weeks and before the actual convention. Currently, it looks like the Dem delegations are lining up behind VP Harris so I guess we'll just have to see.