And they will. We have to tread carefully where Trump is concerned. His base is already sensitive to any criticism, and quite frankly many have been royally screwed by both parties. Theirs is an emotional response based on very real concerns that will only get worse the more they perceive that he is being challenged. This is real Old Testament "eye for an eye" territory. Not a criticism, but an observation.
The two parties along with the corporate media created this train wreck of a campaign. What if the DNC and the mediahad played fair with Sanders whose positions on trade, finance, social spending, etc, might have excited some of the very people now flocking to Trump? There would always be a contingent mesmerized by his celebrity no matter what he said or did, but he is speaking some truth to these folks. And again, since the media has since JFK focused on style and not content, the electoral process has been reduced to a reality show.
It's really scary to note that Trump's not the worst candidate out there (see Ted Cruz).
I find it interesting that Trump is now targeting Sanders - which means his campaign may now see him as a real threat. Trump believes, and may be right, that he can handily defeat HRC.