A handshake agreement of sorts has kept Democratic candidates from going for the jugular in political attacks against one another. It’s a code that Bernie Sanders has expressly vowed to stick to for the entirety of the presidential race and a cause that Hillary Clinton has supported in steering full attention to the real issues of the race.

The candidates tested the boundaries of that agreement this week in a spat over whether Clinton’s progressive bonafides are genuine. But the dynamic took a turn during Thursday’s MSNBC debate into uncharacteristically combative territory.

Caught on the defensive over her progressive credentials, Clinton returned fire by casting doubt on Sanders’ own integrity. Clinton called out his sweeping condemnations of corporate influence on politics as veiled personal attacks against her character.

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Her opening came early in the night, just after Sanders took subtle jabs at establishment candidates – no names named – who for years have raised money from drug companies and other special interests.

CLINTON: I think it’s fair to really ask what’s behind that comment. You know, Sen. Sanders has said he wants to run a positive campaign. I’ve tried to keep my disagreements over issues, as it should be.

But time and time again, by innuendo, by insinuation, there is this attack that he is putting forth, which really comes down to – you know, anybody who ever took donations or speaking fees from any interest group has to be bought.

And I just absolutely reject that, senator. And I really don’t think these kinds of attacks by insinuation are worthy of you. And enough is enough. If you’ve got something to say, say it directly.

…

CLINTON: So I think it’s time to end the very artful smear that you and your campaign have been carrying out.

It was clearly plotted out to be her knockout punch. Throughout the evening, Clinton had been setting up the moment by chipping away at Sanders’ credibility as a candidate, without saying it head on. But the small jabs looked strikingly similar to the very thing she was denouncing: attacks laced with innuendo. And her doing so only helped cement the heart of Sanders’ critique of her campaign, that she is deeply ingrained into the system that he is rebelling against.

At every opportunity, Clinton peppered her responses with the voices of others, and not just herself, who felt Sanders’ policy proposals were unrealistic. She referenced the national security and military intelligence experts who were concerned about Sanders’ view on foreign policy. She cautioned that “every expert” she talked to questioned how to control costs with a free college system, and that a “respected health economist” said Sanders’ single-payer health care plan would cost $1 trillion more a year.

Clinton went on to name-check the many credible experts who supported her ideas. There were progressive icons Paul Krugman and Barney Frank, who side with her on Wall Street reform, as well as the “independent experts” and “editorial boards” that back her, not to mention the many elected officials from Sanders’ own state of Vermont who see her as the most viable candidate against the eventual Republican nominee in November.

“I am not going to make promises I can’t keep,” she said to drive the point home.

Close video Clinton may release transcripts of speeches Presidential candidate Hillary Clinton says she will look into releasing the transcripts from the speeches she gave on Wall Street. Presidential candidate Hillary Clinton says she will look into releasing the transcripts from the speeches she gave on Wall Street. share tweet email Embed

“You will not find that I ever changed a view or a vote because of any donation that I ever received,” she said Thursday night. But when asked if she would release the transcripts of her speeches before Goldman Sachs execs, which earned her upward of $675,000 in speaking fees, Clinton said she’d “look into it.”

Sanders had been hitting at that message in a series of ads out last month that attacked financial institutions for handing over cash and speaking fees to candidates. Based on his response Thursday, it’s a line of messaging that he plans to keep up.

“One of the things we should do is not only talk the talk, but walk the walk,” he said. “I am very proud to be the only candidate up here who does not have a Super PAC, who’s not raising huge sums of money from Wall Street. And special interests.”