“Reconciling his differing public personae will be crucial in attaining such a balance.”
Actually, Trump just needs to be careful that he is always right when he pops off.
In this case !Yeb! has so few supporters and so widely disliked the observation is almost indisputable. Who the Hell cares? Making fun of Jeb won’t cost Trump any votes.
The candidates all did pledge to back the nominee whoever it would be. A few of these characters are not holding up to their pledge. Jeb is one of them so Trump has every right to point this out. If one can’t hold up to a pledge like that how can you expect him to hold up a pledge to the US as president.