“You’re looking at someone who has never run a negative TV ad in his life and never will,” he said. “There are sometimes there are gray areas, we felt that it was a gray area, we took the ad down.”

The Vermont senator’s communications director Michael Briggs characterized the “small digital ad buy” as “a miscommunication in our communications shop,” and disputed the idea that it was a full-blown attack ad.

“We make comparisons,” he said of his team’s strategy regarding Hillary Clinton.

Sanders has made the source of donations a frequent point of comparison over the last few months.

“I am the only candidate running for the Democratic nomination who does not have a Super PAC,” he boasted to New Hampshire community college students on Monday. He noted his campaign is largely bankrolled by small donors, with an average contribution of just over thirty dollars.

The tone of the Democratic race has largely been a civil one, though Martin O’Malley has begun calling his rivals out by name leading into Saturday’s debate on ABC.

Sanders, meanwhile, said his tone on the airwaves won’t change.

“We are running a positive campaign,” he said. “Now you ask any other candidate out there if they’ve ever run negative television ads. Not one of them will say they haven’t. I have not.”

His campaign denied there any other negative ads waiting in the wings should the nature of the race change.

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