1. Donald Trump can’t really be the nominee, because his candidacy is a bizarre and unsustainable media phenomenon.
2. Ted Cruz can’t really be the nominee, because he is incredibly loathsome to the normal human beings who vote for president.
3. Everyone else who could be the nominee is currently being beaten by Donald Trump and Ted Cruz.
This is the Escher staircase that Jeb Bush is trudging up. He has to believe he can’t lose to Trump. He has to believe he can’t lose to Cruz. So he can’t believe that he can’t win.
My guess is still that Rubio will to some extent break the knot by finishing ahead of Cruz and clearly ahead of Christie and Jeb! in New Hampshire. Things are at least trending in a “Party Decides” direction. But given how long Rubio has run behind Cruz and Trump, considerable skepticism about his candidacy remains justified.