This post has been updated with comments from the Clinton campaign conference call.

In what is by far his most toughest TV ad yet, Bernie Sanders took a not-so-subtle swipe at rival Hillary Clinton’s Wall Street donations in a new spot that will soon be broadcast in Iowa and New Hampshire.

“There are two Democratic visions for regulating Wall Street,” Sanders says, looking straight at the camera. “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. Then we can expand health care to all and provide universal college education,” he continues. “Will they like me? No. Will they begin to play by the rules when I’m president? You better believe it.

Sanders and his campaign have been knocking the Democratic front-runner over her donations from the financial sector for months. “Our opponents have been running TV ads since early summer. They’re funding their campaigns with checks from billionaires and Wall Street,” reads a fundraising email sent under Sanders’ name from November.

RELATED: Clinton ramps up attacks on Sanders

But broadcasting a paid TV ad with that message is a major escalation, and a potentially risky one for a candidate who has vowed not to go negative. “You’re looking at someone who has never run a negative TV in his life and never will,” Sanders told ABC News last month.

The new spot, which does not mention Clinton by name, is not a negative ad. Still, it draws an obvious implicit contrast and is the closest Sanders has come to running a real attack ad during his presidential campaign.

Sanders spokesperson Michael Briggs said the ad is not “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,” he said in a statement to MSNBC. “Obviously she is part of the establishment that Wall Street has showered with financial support. Bernie is not. She wants Wall Street and corporate special interests to like her. Bernie does not.”

Last month, Sanders’ campaign abruptly took down and disowned a digital advertisement that said Hillary Clinton was “bank-funded” while Sanders was “people powered.” “We haven’t been doing ads that mention Hillary Clinton,” Briggs told the Washington Post at the time, chalking up the existence of the ad to a “a miscommunication” among the campaign’s staff.

Still, the the campaign has posted non-sponsored content to its Facebook page with messages like, “Hillary Clinton May Have Wall Street, But Bernie Sanders Has You.” During the first two Democratic debates, Sanders’ spokesperson sent opposition research to reporters noting that Clinton’s top lifetime donors include Citigroup, Goldman Sachs and JPMorgan Chase.

The former secretary of state has ran her own advertisement this week drawing an implicit contrast with Sanders. Timed to coincide with President Obama’s final State of the Union Address Tuesday night, her campaign released an ad featuring Clinton speaking to the camera on gun control. “It’s time to pick a side. Either we stand with the gun lobby or we join the president and stand up to them. I’m with him,” she says.

Although it makes no reference to Sanders or two different approaches to gun control, the message is clear for anyone following the campaign, as Clinton has accused Sanders of siding with the National Rifle Association when he voted against the Brady Bill and for immunity for gun sellers.

On a conference call with reporters Thursday afternoon — the Clinton campaign’s third conference call in the past week targeting Sanders — senior officials accused Sanders of violating his pledge not to run negative ads.

Clinton chief strategist Joel Benenson said Sanders had “decided to do something that he had said so proudly he would never do.” But he did not rule out the Clinton campaign running negative advertisements of its own. “I think we will wait to see what Sen. Sanders does,” Benenson said.

Clinton campaign manager Robby Mook said Sanders’ commercial was “clearly a negative ad,” adding that it was completely different from his own campaign’s gun ad released this week. Mook did not provide specific definition for a negative ad, but said his own ad was about policy, made no mention of Sanders, and that there was no attack on her rivals character. “You really can’t compare the two,” he said.

Clinton began airing television ads months before Sanders, but he is now spending more week-to-week than her on advertising. Clinton’s campaign has spent a total of $14.6 million on TV ads compared to Sanders’ $10.3 million.

Sanders spent $1.4 million last week compared to Clinton’s $1.2 million for ads in in Iowa, New Hampshire, South Carolina and Nevada, according to an ad buying source.