I read this article today http://www.politico.com/magazine/story/2016/01/how-to-take-trump-the-bully-down-213536 and thought it was fantastic and pretty accurate. Bullies do operate and win by caricaturing a weakness. Nobody wants to deal with them because there is that kernel of truth they are raising, and also because if you swing at the bully and don't draw blood, you make the taunts look that much more accurate. Cruz is kind of hanging in against Trump I believe because Cruz does seem to genuinely believe he is truly that much better and that much more correct than anyone else on the stage. When Trump hits the others, they respond and typically respond in a way that makes you think they might have been better off to just ignore the jab.