At the Washington University debate Sunday night, Donald Trump threw out a serious threat to political opponent Hillary Clinton.

>> Watch the moment here

When the subject of the 2005 video of Trump saying lewd things about women and the need for an apology arose, Trump responded by saying Clinton should be the one apologizing for her email scandal.

“But when you talk about apology, I think the ... thing that you should be apologizing for are the 33,000 e-mails that you deleted and that you acid-washed, and then the two boxes of emails and other things last week that were taken from an office and are now missing,” he said.

>> Donald Trump holds pre-debate news conference with Bill Clinton accusers

Trump went further about what he would do, should he become president.

"If I win, I am going to instruct my attorney general to get a special prosecutor to look into your situation because there has never been so many lies, so much deception," he said. "There has never been anything like it. And we're going to have a special prosecutor."

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Clinton urged viewers to fact-check Trump's statements, adding, "You know, it is just awfully good that someone with the temperament of Donald Trump is not in charge of the law of the country.”

“Because you’d be in jail,” he replied as several people in the audience cheered.