Let's jump to early August ...

[pbrower2a]

The assassination of Robert Kennedy in 1968 answered that question for Democrats: delegates for a deceased candidate are then released.

Such would be an incalculable disaster for the Party whose most likely nominee is no more. That Party all of a sudden has a candidate usually ill-vetted to satisfy the factions within the Party and has inadequate time in which to develop a strong campaign. Hubert Humphrey was by all accounts a fine person who would have been a good President -- but he was unable to keep the segregationist wing of the Democratic party from seceding.

Richard Nixon may have been the craftiest Presidential nominee that we ever had, so he might have won anyway. I can't be sure that Robert Kennedy would have stopped the Wallace secession. But just figure that Kennedy might have done better in enough states to cut into some Nixon majorities that resulted from people wasting votes by choosing Wallace in states that Wallace did not win. Illinois, Missouri, Ohio, and Wisconsin were close enough to be decided by the Wallace vote; take away half the Wallace vote in Florida and Kentucky, and the Democrat wins.

That said, Richard Nixon ran as visceral a campaign as anyone ever did and well hid the political demons that would eventually bring him down.