Hillary! spent 8 years in the White House as second banana already. If that memory doesn't stick with you, it certainly is seared into the consciousnesses of her core supporters.
They won't tolerate Hillary! running for a position which requires a microsecond more of the WH second-banana position.
It's more than political to them, it's assuredly personal.
They also consciously recognize McCain for what he is, a political turncoat opportunist with no core belief system.
If McCain can betray the Republicans so frequently and so publicly, what could ever be perceived as restraining him from betraying his "friends" on the other side of the aisle later on, as President-- or reneging on any presidential campaign pre-nup agreement with Hillary!, for that matter?
Besides, he is a white male, which is to say, he is considered equivalent to subhuman among Hillary!s core supporters-- no matter what he professes to support or not support politically at any given time.
You're not thinking like a democrat. They want a win. John can provide that. Hillary is iffy.
You don't get it. John won't jump parties unless he's assured of the DNC's backing.