Now, a lot of Trump’s support is for Trump, not against Bush. It has morphed over time. But there is still a strong contingent who believe in the binary — if you are not for Trump, you are for Jeb Bush. I’m sure if Bush walked away, it would become against Rubio. The Trump folks seem to recognize this and have spent a lot of time bashing Rubio lately.
Still, I wonder what would happen if Bush walked away. The odds are he does not, but the odds are also that as long as Jeb Bush hangs on, he prevents a necessary shake up in the race for those who are anti-establishment to look again at the field. There is a psychological hold on a lot of the anti-establishment forces convinced that Bush, by virtue of his last name and perceived second coming of Romney, will walk away with the nomination if left unchecked.
If Jeb Bush goes away, so too do many of those psychological holds, barring a Bush endorsement for someone else.