Well he's on his heels a little bit because of the "qualified to be President" boondoggle. He read an article that was headlined "Hillary questions Bernie's qualification to be President" and retaliated by stating the she was not qualified to be President. Then when it came out that she never said that, and the headline was just sensationalizing her campaign essentially saying that they were going to start attacking him, and try to disqualify him and defeat him in the minds of voters. So then he had to backtrack and actually admit that "Of course she is qualified to be President." The corner that Bernie is in, is that Hillary has so much to attack, but he tied his hands behind his back by "taking the high road." It got him a ton of goodwill early on, but he has basically ridden that donkey for all its worth . Hillary saw the corner he was boxing himself into, and she just went along with it... "Oh you want to stay positive? Ohhhh Kaaaay fine, we can do that" because she knew that in the end staying positive benefits her, probably more than him. Now is the time where he needs to pivot to attacks and negative ads, but he can't, because he built this huge house of cards on the illusion that he is "different" from "typical" and "establishment" politicians and that he "sticks to the issues"... its his whole brand. So now he is desperately trying to bait her into "provoking" him so he has some moral high ground to go negative. He said (paraphrasing) "If they think we small town Vermonters don't know how to go negative... we can learn fast", again, implying that he will only do it if "she starts it." Please you wish. Why would Hillary accept that bait? She has him right where she wants him, trapped in the corner that he painted himself into. He needs to go negative to catch up, but he can't unless she does first... which she won't because she doesn't need to when she's already winning.