Sanders, however, denies the ad —- which does not mention Clinton by name —- is about her.
The ad in question is titled “Two Visions” and was released earlier today for air in Iowa and New Hampshire.
In the ad, Sanders says: "There are two Democratic visions for regulating Wall Street. One says it's okay to take millions from big banks and then tell them what to do. My plan: break up the big banks, close the tax loopholes, and make them pay their fair share.”
The Clinton campaign held an abruptly scheduled conference call with reporters this afternoon to respond to the ad.
“We were very surprised today to see that Bernie Sanders had launched a negative television advertisement against Hillary,” Clinton campaign manager Robby Mook said.
“This ad does something that Bernie said he wouldn’t do,” campaign pollster Joel Benenson added, referring to how the Vermont Senator has said he will not run negative campaign ads.
During a press conference tonight, Sanders denied the Clinton campaign's assertion that this line was an attack on Clinton.
During the conference call, the Clinton campaign denied the gun ad was going after Sanders, but also did not rule out the possibility of launching negative ads against him in the future.
"I think we will wait and see what Senator Sanders does,” Benenson said.
The back-and-forth between the two candidates comes the same week new polls show Sanders gaining on Clinton in the polls, including a Quinnipiac poll of Iowa that has Sanders with a 5 point lead.
ABC News' MaryAlice Parks contributed to this report.