Donald Trump's pledge Sunday night that he would order his attorney general to investigate Hillary Clinton, and his quip that she should "be in jail," is a direct breach of the tradition of nonpartisan rule of law.
"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, there has never been anything like it," Trump said during the second presidential debate.
A president is not typically authorized to order specific criminal investigations of individuals, let alone a public pledge to investigate a political opponent. Former Attorney General Eric Holder tweeted that President Richard Nixon's attorney general "courageously resigned" after being asked to fire a special prosecutor investigating the Watergate scandal.
When Attorney General Elliot Richardson refused, Nixon went on to fire several members of his administration in what became known as the "Saturday Night Massacre."
The FBI and Department of Justice have formally closed the inquiry into Hillary Clinton's use of a private email server as secretary of state. So the notion of a new president seeking to force the re-opening of the case, because a new party is in office, is essentially unprecedented.
Also note that while Trump has previously talked about investigating Clinton on the campaign trail, including discussing the statute of limitations for charges related to the email issue, his language then was less definitive than what he said Sunday night.
In July, he said he expected "the attorney general will take a very good look at it, from a fair standpoint," referring to the email inquiry.