Either Kerry would step aside when the Klintoons make it clear to him that he has no choice, or the 'Rat party leadership comes out against him because he has become totally unelectable, and the 'Rat Convention Delegates revolt en masse, to choose someone who can be elected.
If Kerry's election hopes were sufficiently toast before the Convention began, then I am confident he would not be the 'Rat party candidate for President.
One must never forget New Jersey's bait and switch in 2002, from Toricelli to Lautenberg in a couple days.