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“I think, in fact, he’s probably running the most negative campaign of any Democratic presidential candidate … in a presidential primary season, yes,” said Joel Benenson, Clinton’s senior campaign strategist.

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“He’s running fundamental attacks and he’s going out on the stump and raising issues about her personally and her character and [that] of other Democrats as well,” he continued on CNN.

“I think he’s kind of ‘feeling the Bern’ as he’s getting more scrutiny,” Benenson added, mocking a slogan used by supporters of the Vermont senator.

“It seems like once you’re not with him, you become a vocal point of his attacks. I do think it’s been extremely negative and I think it’s unfortunate.”

Benenson, formerly a top pollster to President Obama, said he doesn't recall the 2008 race being as contentious.

“I was on [President] Obama’s side then,” he said. "I don’t think we had the range of negativity on either side.”

Sanders has repeatedly vowed his campaign would reject personal attacks.

He is sharpening his criticism of Clinton, however, as he runs close with the Democratic front-runner in crucial early voting states.

Clinton leads Sanders by only 2.5 points in Iowa before its first-in-the-nation caucuses on Monday. He has a solid lead in New Hampshire, holding a 14-point lead over Clinton in the state.

The former first lady has the edge nationwide, however. Clinton has a nearly 15-point lead over Sanders across the country, according to the latest RealClearPolitics average of polls.