Republican presidential candidates dominated television sets in Iowa and New Hampshire this past week, but a commercial for Senator Bernie Sanders, “Two Visions,” captured the most attention, largely because of how Hillary Clinton’s campaign responded to it.

Mr. Sanders addresses the camera to the sort of plunking xylophone music often heard in pharmaceutical or insurance ads. “There are two Democratic visions for regulating Wall Street,” he says, over a snail’s-eye shot of soaring Manhattan office towers. “One says it’s O.K. to take millions from big banks and then tell them what to do.” Animated pictograms show a faceless banker in a suit handing over a bag of cash to a couple with two children and a stroller, beneficiaries of the plan he bluntly sums up: “Break up the big banks, close the tax loopholes and make them pay their fair share” to fund “health care for all” and “universal college education.”

Message

A direct contrast with Mrs. Clinton, though she is not named, who is criticizing Mr. Sanders on health care and gun control. Hitting back, he paints Mrs. Clinton’s promises to regulate Wall Street as disingenuous, reminding astute listeners of her millions of dollars in financial-industry donations and paid speaking fees over the years.

Response

As the ad was beamed into homes in Iowa and New Hampshire, the Clinton campaign cried foul, calling it a negative attack and a violation of Mr. Sanders’s high-minded pledge never to run such an ad, ever. Mr. Sanders and his team denied this characterization. Instead, they pointed to a Clinton commercial a day earlier that, without naming Mr. Sanders, suggested that he stood with the firearms industry and against both Mrs. Clinton and President Obama on the subject of expanded gun control laws.