That's true. The closest thing I could find to a traditional negative ad from Sanders was this one that hits Wall Street for its campaign contributions and speaking fees, and, by association, Clinton.
This one, in which Sanders lays out "two visions" for dealing with the economic inequality, is also kind-of, sort-of negative. But not really.
Ads aside, Sanders has totally avoided talking about Clinton's private email server and the FBI and State Department investigations into it — insisting that "no one cares about your damn emails" even while polling suggests lots of people do.
In a genuinely negative or nasty campaign, Sanders would have spent most days this year slamming Clinton as dishonest and untrustworthy for her initial decision to set up the private email and for her subsequent insistence that the server never needed to be turned over to a third party investigator. He could be saying that Clinton represents too big a risk for Democrats to nominate because of the investigations looming over her.
Sanders has done none of that. But, he doesn't tell people not to boo Clinton at his events! THE HORROR! WHY SO NEGATIVE!!!
Meanwhile back here on Planet Earth, it's clear why Clinton's team isn't super interested in debating Sanders in New York — or maybe anywhere else. She is winning the delegate race. And, barring some sort of catastrophe, she will continue to win it.
Why, they think, give Sanders a high-profile platform — particularly in New York where he can bash her on ties to Wall Street — to try to force a Clinton stumble? Running out the clock can and often does work. (There's a reason Dean Smith's "Four Corners" offense worked so well.)
While I tend to disagree with that strategic calculation — Clinton is a remarkably able debater and almost always wins these things — I can understand her campaign's resistance to more debates, particularly if it looks like they are allowing Sanders to dictate the terms of engagement to them.
But, what makes zero sense is to say that the real reason that Clinton isn't interested in debating is because Sanders has been too negative. This campaign has been one of the least contentious I have ever covered and stands in marked contrast to the mudfest on the Republican side. Clinton has every right to refuse further debates but citing Sanders's negativity to explain it away doesn't fly.