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In the pantheon of campaign attack ads, this one seemed gentle enough: Senator Bernie Sanders explains his intention to regulate the financial industry, including breaking up the big Wall Street banks.

“There are two Democratic visions for regulating Wall Street,” he says amid stock images of faceless bankers, bags of money and a wholesome family. “One says it’s O.K. to take millions from big banks and then tell them what to do.”

He never mentions his main Democratic rival, Hillary Clinton, but her campaign said he might as well have.

On a conference call with reporters, Mrs. Clinton’s senior campaign aides expressed outrage over the ad, which will air in Iowa and New Hampshire and which Clinton aides said violated Mr. Sanders’s pledge to never run a negative attack ad against a political opponent.

“You’ll have to ask Senator Sanders — he’s in this ad — why he decided to do something he so proudly said he’d never do,” said Joel Benenson, Mrs. Clinton’s chief strategist and pollster.

Robby Mook, the campaign manager, echoed the sentiment. “We were particularly surprised today to see them break that pledge and run this negative ad,” he told reporters.

Last month, the Sanders campaign promptly pulled a digital ad that referred to Mrs. Clinton as “bank funded,” and his aides said the negative ad had been the result of a “miscommunication.”

The Sanders ad comes after Mrs. Clinton has intensified her attacks on his health care proposals and record on gun control legislation.

In an ad, broadcast in Iowa and New Hampshire on Tuesday night during the State of the Union address, Mrs. Clinton seemed to draw a contrast with Mr. Sanders on gun control, without mentioning him.

“It’s time to pick a side,” she said. “Either we stand with the gun lobby, or we join the president and stand up to them. I’m with him.”

Mr. Mook said the ad was completely different from Mr. Sanders’s Wall Street ad.

“It made absolutely no mention of Senator Sanders, implicitly or otherwise,” he said.

Republicans were quick to accuse the Clinton camp of an inability to handle Mr. Sanders’s kid gloves.

“If the Clinton campaign is too thin-skinned to handle velvet-gloved contrasts from Bernie Sanders, you’ve got to wonder how they’re going to get through a general election,” said Michael Short, a spokesman for the Republican National Committee.