The Democratic frontrunner’s campaign organized a news conference call with reporters on Thursday, following the release of a new ad by the Bernie Sanders campaign that it deemed to be “negative”. Recalling Sanders’s pledge not to run negative advertising, Hillary Clinton’s campaign was somewhat outraged by what they said was an obvious change in tone.

“We were very surprised today to see that Bernie Sanders launched a negative television advertisement against Hillary,” Clinton campaign manager Robby Mook said.

Campaign strategist Joel Benenson chimed in that the ad “does something that [Sanders] so proudly said he wouldn’t do”.

The ad itself is, by most accounts, rather mild. It features Sanders, a senator from Vermont, speaking straight to the camera and painting a contrast between “two Democratic visions” for regulating Wall Street.

“One says it’s OK 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,” Sanders says in the ad.

At no point does he mention Clinton by name, nor does the ad feature an image of the former secretary of state.

Reporters pressed the Clinton campaign several times during the call on whether they truly felt the ad was “negative”, to which both Mook and Benenson reiterated that it violated, in their view, Sanders’s vow not to run attack ads.

Asked if the Clinton campaign would respond in kind, they said they would “wait and see what Senator Sanders does”.

The primary fight between the two campaigns has grown increasingly heated in recent weeks, as Clinton and Sanders remain locked in a competitive race in the early voting states of Iowa and New Hampshire.

Clinton’s campaign has aggressively attacked Sanders over his record on gun control and released a new ad itself drawing a contrast on the issue. Clinton similarly speaks to the camera and, while not invoking Sanders’s name, says it’s “time to pick a side” between the gun lobby and those standing up against it.

