Hillary Clinton campaign strategist Joel Benenson made an eyebrow-raising claim Friday on CNN, accusing Sen. Bernie Sanders (I., Vt.) of running the most negative Democratic primary campaign in history.

Sanders has aired ads attacking Wall Street and big banks without naming Clinton, who has received huge speaking fees from Goldman Sachs, but he has also made it clear he would not engage in personal mud-slinging during the campaign.

"I think he's going negative," Benenson said. "I think he's probably running the most negative campaign of any Democratic presidential candidate."

"You think so?" anchor Kate Bolduan asked.

"I think so, in a presidential primary season, yes," Benenson said. "I think he's been more personal in his attacks. I think he's been increasing it on the stump recently, and I do, I can't think of one. Even in a very hard-fought campaign in 2008, I don't think we had the range of negativity on either side, and I was on Obama's side then, that we've had now."

Bolduan pointed out to Benenson that Sanders helped Clinton by dismissing her private email controversy during the first Democratic debate in October, famously remarking that the American people were sick of hearing about "your damn emails!" Sanders also has said, while noting Bill Clinton's past sexual misconduct was "deplorable," that Hillary was his true opponent and her husband's past malfeasance wasn't relevant to their political debate.

Sanders also said during that exchange at the NBC debate that he had "avoided" attacking Clinton, to which Clinton herself nodded and smiled.

Benenson stood firm in his claim against Sanders.

"He's running fundamental attacks, and he's going out on the stump every day raising issues about her personally, her character, and other Democrats as well," he said. "Democrats, by the way, who enacted the toughest rules on Wall Street and the banking system in seven decades."

In the last three weeks, Benenson said, Sanders himself might be "feeling the Bern" because of the increased scrutiny he's gotten as he's caught up with Clinton in the Iowa polls.

"It seems once you're not with him, you become a focal point of his attacks, and I do think it's been extremely negative," Benenson said. "I think it's unfortunate."

Clinton's campaign was tied to spreading the false rumor that then-Sen. Barack Obama was a Muslim when the two were battling for the 2008 Democratic nomination. When pressed on the subject at the time, Clinton remarked "as far as I know" there was no basis to the Muslim rumors about Obama.