Rush wasn't criticizing Trump for going negative against Cruz. It was the issue. Trump was criticizing Cruz for not playing ball with the Senate leaders.
Trying to explain “why” to Cringer is just a waste of time. He/she doesn’t think in terms of why.....
Cruz should very nicely respond something like this:
I like Donald Trump. I wish every American businessman were as successful as Donald Trump. We agree on many issues, disagree on a few. I am not going to attack him and get in a fight with him or any other republican candidate.
But Donald criticizes me for being unable to make deals. Well, it’s just this kind of deal making that got us in trouble. We don’t need deal-makers like McConnell, Boehner, and Ryan; we need someone with a proven record of standing up for the principles and values of the American people and the Constitution.