's campaign on Thursday decried what it called an attack ad from Democratic presidential rival Sen.(I-Vt.).

Clinton's aides claimed Sanders had broken his pledge to never run a negative advertisement by releasing his 30-second campaign spot on "two Democratic visions for regulating Wall Street."

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"We were very surprised today to see that Bernie Sanders had launched a negative television advertisement against Hillary," Clinton campaign manager Robby Mook told reporters on a phone call.

"We were particularly surprised because he had personally pledged, and his campaign had pledged, never to run a negative advertisement," Mook added, arguing Sanders did "break that pledge."

Sanders's campaign insisted that the ad wasn't "directed at Secretary Clinton exclusively." "It's about people in the Democratic establishment who believe you can take Wall Street's money and then somehow turn around and rein in the greed, recklessness and illegal behavior," Sanders campaign spokesman Michael Briggs said in a statement shared with The Hill. "It's about people in the Democratic establishment who believe you can take Wall Street's money and then somehow turn around and rein in the greed, recklessness and illegal behavior," Sanders campaign spokesman Michael Briggs said in a statement shared with The Hill. "Obviously she is part of the establishment that Wall Street has showered with financial support. Bernie is not," Briggs added, before touting the number of small-dollar donors contributing to his campaign. "Obviously she is part of the establishment that Wall Street has showered with financial support. Bernie is not," Briggs added, before touting the number of small-dollar donors contributing to his campaign.

Clinton and Sanders have been sharpening up their rhetoric, feuding over guns and healthcare as Clinton seeks to fend off an upset in Iowa and New Hampshire early next month.

The former secretary of State's aides said Thursday that the latest Sanders ad signaled what they believed to be a new phase of the campaign less than three weeks until ballots are cast in the Iowa caucuses, on Feb. 1.