Actually, I'm of the opinion that Jeb could very well lose the nomination to McCain. A big reason he lost in 2016, aside from the split establishment vote, was that he was unwilling "to tear the other guy down" (his words) in order to get to the top. Dubya was nominated in part because he waged a bloody, desperate smear campaign against McCain in South Carolina after losing NH. When McCain brought this up in a debate, Dubya humiliated him and from then on the state was his. Jeb Bush wouldn't do either of these things, meaning he might lose the South Carolina Primary and McCain picks up momentum from there.