The weekend before he raised his right hand in front of the Senate Watergate Committee and swore to tell the truth about President Richard M. Nixon’s crimes, John Dean got a haircut.

“Cut it nice and clean,” Dean told the barber, according to a book he later wrote.

The barber had no idea that the hair he was clipping belonged to the former White House counsel, the man who helped cover up the Nixon campaign’s break-in of the Democratic National Committee’s offices.

“What do you think of these Watergate hearings?” the barber asked.

“They’re pretty interesting,” Dean said.

The country was riveted. People watched at work, in department stores — any place they could find a television. The barber planned to bring a TV to his shop during the upcoming week to watch Dean, who had flipped on Nixon and become a villainous character to Republicans.

“We’ll find out what the squealer has to say for himself,” the barber said.

Dean played along.

“Right,” he said. “You know, I can’t imagine a guy lying that way about President Nixon. The guy is crazy, maybe?”

If that’s what much of the country thought of Dean, that would all change after he methodically detailed his role in the coverup, how it worked and — most important — whether Nixon knew about it.

He did, of course.

Dean’s testimony about Nixon’s abuse of power hastened the president’s demise. Now, President Trump is attacking Dean on Twitter as he testifies in House hearings Monday.

Dean testified there were “remarkable parallels” between the Mueller report and the Watergate investigation.

He was part of a panel of experts, but his was the big name in lights — just as more than 40 years ago, when he appeared on televisions across the country in a tan summer suit and horn-rimmed glasses, and with a fresh haircut.

In 1973, Dean sat alone at the witness table, a calculated move to make clear he was speaking on his own. He had prepared his testimony for weeks, beginning with a 245-page opening statement that took almost an entire day to read. As the senators settled into their seats, Dean tried to make a joke and lighten the mood.

“I sincerely wish I could say it is my pleasure to be here today, but I think you can understand why it is not,” Dean said.

Nobody laughed.

Sam Dash, chief counsel to the Senate Watergate Committee, said, “Mr. Dean, could you please take the microphone and put it closer so we can all hear it?”

Dean did. And then he began:

It is a very difficult thing for me to testify about other people. It is far more easy for me to explain my own involvement in this matter. The fact that I was involved in obstructing justice. The fact that I assisted another in perjured testimony. The fact that I made personal use of funds that were in my custody. It is far easier to talk about these things myself than to talk about what others did. Some of these people I will be referring to are friends. Some are men I greatly admire and respect.

Dean went on. And on. And on. He detailed the shredding of documents. He spoke of “laundering” money.” He used phrases related to organized crime, such as “deep-sixing” a briefcase of cash. And he delivered phrases that have endured in history — particularly “a cancer on the presidency,” stemming from a meeting he held with Nixon, hoping the president would end the coverup and come clean.

I began by telling the president that there was a cancer growing on the presidency and that if the cancer was not removed that the President himself would be killed by it. I also told him that it was important that this cancer be removed immediately because it was growing more deadly every day. I then gave him what I told him would be a broad overview of the situation and I would come back and fill in the details and answer any questions he might have about the matter.

Nixon, of course, did not come clean.

Dean’s testimony was damaging in other ways. He left bread crumbs in his testimony for questions he hoped the committee would ask other witnesses, including about the existence of an Oval Office taping system. Dean told an anecdote about how during one meeting it felt as if Nixon was trying to get him to say specific things.

Then something really weird happened, Dean recounted in his testimony:

… very near the end, he got up out of his chair, went behind his chair to the corner … and in a nearly inaudible tone said to me he was probably foolish to have discussed Hunt’s clemency with Colson. I do not recall that I responded. The conversation ended shortly thereafter.

The implication of this testimony: Nixon had a taping system.

The implication of a taping system: The entire coverup was on tape.

A month later, Alexander Butterfield, Nixon’s deputy chief of staff, testified before the committee. He was asked point blank: Was there a recording system in the White House? Yes, he said. And the phones? Yes, he said. And this recording took place all the time? Yes, he said.

“One last question,” Dash said. “To reconstruct the conversations at any particular day, what would be the best way to reconstruct those conversations, Mr. Butterfield, in the President’s Oval Office?”

“Well,” Butterfield said, “in the obvious manner, Mr. Dash, to obtain the tape and play it.”

Nixon fought the release of tapes all the way to the Supreme Court, which ultimately ruled unanimously that he must.

Two weeks later, he resigned.

Gillian Brockell contributed to this report.

Read more Retropolis: