David M. Dorsen

Opinion contributor

In June 1973, John Wesley Dean III, former White House counsel under President Richard Nixon, transfixed the nation with his one week of testimony before the Senate Watergate Committee chaired by Sen. Sam Ervin. Dean's testimony about his and Nixon’s obstruction of justice led to Nixon’s resignation just more than a year later; he was the first U.S. president to do so.

Some have touted Michael Cohen, President Donald Trump’s onetime lawyer and fixer, as a potential present-day John Dean, capable of bringing down another president. He'll get his first public opportunity Feb. 7, when he testifies before a House committee, and he may eventually appear in court. But before we put too many eggs in the Cohen basket, we need to contrast the respective situations of Dean and Cohen.

The Republican break-in of the Democratic National Committee headquarters in June 1972 resulted in the convictions of its immediate perpetrators, G. Gordon Liddy and Howard Hunt, two journeymen spooks, along with a handful of Cuban nationalists who sought to overthrow Fidel Castro. Six months later, we were no closer to the truth that Nixon and his top White House lieutenants had masterminded the cover-up of a wide range of their misconduct — including breaking into the Watergate and the office of a psychiatrist who treated Daniel Ellsberg, the author of the Pentagon Papers on the Vietnam War.

Read more commentary:

Ex-GOP congressman to Dems: Gear up to impeach Trump. It's your constitutional duty.

Why Donald Trump won't be impeached. Look to Richard Nixon and Bill Clinton.

How House Democrats can save democracy and the rule of law: Impeach Trump ASAP

But the landscape changed when U.S. District Judge John Sirica, who presided over the trial of Liddy, Hunt and the Cubans, challenged the White House version of events and made clear he thought the actual leaders of the operation were far senior to those convicted. Sirica’s insistence led to the creation of the Senate Watergate Committee to get to the bottom of things.

This committee began its work in March 1973. I was assistant chief counsel under Chief Counsel Sam Dash. We were fixed on the convicted felons’ immediate superior, Jeb Magruder, deputy director of the Committee to Re-elect the President (also known as CREEP). He was several levels below Dean and his superiors, including Chief of Staff H.R. Haldeman, Domestic Adviser John Ehrlichman and, of course, their boss, President Nixon.

Dean was a fantastic witness who came to us

Then, amazingly, John Dean walked in the Watergate Committee’s door, literally. At that time, we had no evidence that Dean had done anything wrong. If he had simply kept his mouth shut, I doubt that we would ever have made a case against him. He outlined a conspiracy to obstruct justice, in which he was the daily operations officer overseen by Haldeman, Ehrlichman and, ultimately, Nixon. He testified publicly before the committee under a grant of immunity that prohibited the use of his testimony by prosecutors. He implicated himself and his superiors.

While Dean had engaged in an obstruction of justice, he never lied under oath. His photographic memory was corroborated by the later-uncovered infamous Nixon tapes, and his statements also led the Watergate Committee to find the crucial tapes. Although he almost certainly could have escaped prosecution because his Watergate Committee testimony, given under a grant of immunity, poisoned any prosecution, he nevertheless pleaded guilty and served a short jail term. He was a fantastic witness.

Michael Cohen is different. His professional career was studded with illegalities, including paying off women to remain silent in order to secure the election of his sponsor, Donald Trump. He had engaged in criminal conduct for decades. He lied to federal investigators and many others to save himself. He was an officer of the Trump business organization and pleaded guilty to bank and tax fraud in addition to his violations of campaign financing laws.

Cohen is a deeply flawed, suspicious witness

Significantly, Cohen cooperated only after he was cornered and prosecutors had accumulated a mountain of incriminating evidence, including by conducting an almost unprecedented search warrant of the offices of a lawyer. He had protected himself at the potential expense of his client Trump by recording their telephone conversations. Most of all, he had reluctantly come forward only when his back was to the wall and the person on whom he was relying to rescue him, namely Trump with his pardon power, decided to save himself at the expense of Cohen.

If and when Cohen testifies in open court, his credibility will be trashed, unlike Dean’s, who was not shown to have lied to federal agents or elsewhere. There will always be the suspicion that Cohen knows more than he is admitting or saying a little more than he knows.

It is too early to predict the outcome of the scandals and crimes whirling around Donald Trump. One thing, however, seems certain. Michael Cohen is a deeply flawed witness, who will have to be corroborated at every turn. It will be the mountain of documents, including Cohen’s recordings of Trump, that will be the most impressive evidence of any prosecution or impeachment of the president.

David M. Dorsen served as assistant chief counsel of the Senate Watergate Committee, was an assistant U.S. Attorney in the Southern District of New York, and wrote "The Unexpected Scalia: A Conservative Justice’s Liberal Opinions." His most recent book is "Moses v. Trump, a contemporary novel." The writer represented John Dean in a libel suit against St. Martin’s Press, G. Gordon Liddy and others.