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WASHINGTON (CNN) -- When Bernie Sanders launched 2016 presidential bid, he swore off negative campaigning.

Months later, the Vermont senator is signaling that he's increasingly willing to take personal shots at his chief rival in the Democratic primary, a little more than three months from Iowa's caucuses.

In Sanders' view, he's finishing what Clinton started -- first in the opening Democratic debate, when she assailed his moderate record on guns; then in the Iowa Jefferson-Jackson Dinner, where she suggested his criticism of both sides' "shouting" on gun laws was a gender-based attack on her.

Sanders' senior strategist, Tad Devine, says the campaign is reacting to Clinton's attacks.

"If they're going to have a campaign that attacks Bernie on gun safety, and implies he engages in sexism, that's unacceptable," Devine told Politico.

"We're not going to stand for that. We're not going to sit here and let her attack him. We're going to have to talk about other things if they do that," Devine said. "If they're going to engage in this kind of attack, they need to understand we're not going to stand there and take it."

Sanders himself has been more willing in recent days to criticize Clinton by name. His strategy has involved casting Clinton as a late comer to causes Sanders has championed for decades.

Three primary examples: the Trans-Pacific Partnership, which he's opposed for years while Clinton just weeks ago announced her opposition; the Keystone XL pipeline, which he has long fought as Clinton waited for a State Department decision on whether it should go forward; and same-sex marriage.

Sanders has pointed to Clinton's support for the Defense of Marriage Act -- a 1996 law signed by President Bill Clinton, which defined marriage as between one man and one woman -- as evidence of their differences. Clinton supported the law at the time, though she has since changed her stance. Sanders voted against it.

He pointed to gay rights during his own speech Saturday night at the Jefferson-Jackson Dinner, implying -- without using her name -- that Clinton has changed her stances on civil rights issues "just because it is politically expedient at a given time."

It's a departure in tone and tactics from where Sanders began.

"I've never run a negative political ad in my life," he said during an appearance on CNN's "State of the Union."

"I believe in serious debates on serious issues. I've known Hillary Clinton for 25 years. Maybe I shouldn't say this: I like Hillary Clinton. I respect Hillary Clinton," Sanders said. "Will the media, among others, allow us to have a civil debate on civil issues? Or is the only way you get media attention by ripping apart somebody else?"

However, he downplayed the evolution when speaking to CNN after Saturday night's Democratic dinner in Des Moines.

"What a campaign is about is contrast. That it is what it is about," Sanders said. "I don't want negative ads and I don't believe in negative campaigning, but what is the sense in being in a campaign if you don't differentiate your difference with your opponents?"

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