"I am proud that the gentleman who is the head of Goldman Sachs ... said I was dangerous, and he's right. I am dangerous for Wall Street," Sanders said to cheers during Wednesday night's Democratic debate hosted by CNN and Univision, apparently referring to an interview that Goldman CEO Lloyd Blankfein gave to CNBC's "Squawkbox" program.

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Clinton said, as she has often before, that before the financial crisis she went to Wall Street to tell them to stop their reckless behavior.

Sanders mocked her for that claim, saying sarcastically, "Clearly the Secretary's words to Wall Street has really intimidated [Wall Street], and that is why they have given her $15 million in campaign contributions."

The crowd in Miami laughed and applauded.

"Wait a minute," Clinton said. "I just think it's worth pointing out that the leaders of the fossil fuel industry, the Koch brothers, have just paid to put up an ad praising Sen. Sanders."

Clinton was referring to an ad released on Wednesday by Freedom Partners — a top Koch network group — praising Sanders for his opposition to the Export-Import Bank, which the billionaire Koch brothers slam as "corporate welfare" for profitable corporations like Boeing.

"I guess, Sen. Sanders, that the Koch brothers ... are sensible with how they use their money, and I agree with you, they stand for things that I find abhorrent," Clinton said. "But they did just put up a little video praising you."