The Watergate scandal finally came crashing down around Richard Nixon on Aug. 5, 1974. That was the day, 21 months after he’d won 49 states in his reelection bid, that the White House released a transcript of the tape that became known as the “smoking gun.” Twenty-six months after the burglars were caught inside Democratic headquarters, thing had accelerated into saturation news coverage, televised hearings, combative press conferences, indictments, arrests, constitutional crises. He resigned days later.

These days, everything moves faster. Today’s startling news that Donald Trump told FBI Director James Comey to end the investigation of former National Security Advisor Michael Flynn comes less than four months into this president’s first term.

Trump is not the first president to be vexed by Justice Department or FBI investigations. Chief executives don’t like the idea of someone with subpoena power peering into their inner circle and activities. Nixon, Bill Clinton, and George W. Bush, among others, faced high-stakes investigations. How they reacted set the course of their presidencies. Few have handled it worse than Trump.

To set the scene, let’s begin ages ago in our great national lesson in what constitutes “obstruction of justice,” say, five days ago. When Trump fired Comey, there was plenty of evidence—commonsensical, circumstantial, but not quite direct—that the purpose of the firing was to derail a looming and increasingly threatening investigation. Then the president did us all the favor of explaining to Lester Holt that “this Russia thing with Trump and Russia” was on his mind when he decided to fire Comey.

Now comes word of Comey’s extraordinary contemporaneous memo describing a meeting with Trump. The president asked the FBI chief to hang back after a meeting in the Oval Office, the day after Michael Flynn resigned for having lied about his contacts with Russia. “I hope you can see your way clear to letting this go, to letting Flynn go,” Trump told the no-doubt astonished Comey in his written account. “He is a good guy. I hope you can let this go.”

This was blunt and explicit—no hinting inferences here. It’s wildly problematic, of course, for several reasons. Trump is not just some kibitzer or constituent. He was Comey’s boss. “Letting Flynn go” sounds rather, well, literal. Already, in an earlier conversation,, the president had asked for “loyalty.”

If the White House can just order investigators to shut down when they get close to the president, there’s no possible independent check on Oval Office lawlessness. Trump’s words seem to violate one of the federal obstruction of justice statutes, which applies to “whoever corruptly… endeavors to influence, obstruct, or impede the due and proper administration of the law under… any pending proceeding.”

Which is why Comey’s notes compare to Nixon’s “smoking gun” in 1974. The year before, former White House counsel John Dean had riveted the nation when he testified before the Senate Watergate committee. Dean’s most explosive charge was that he had warned Nixon of a “cancer on the presidency,” with continuing demands for hush money by the burglars. Nixon claimed he had told Dean “that would be wrong.” Then came the revelation that Nixon taped his own Oval Office conversations—thus the tapes would show who was telling the truth. For a year, the president, the prosecutors and Congress battled over the tapes. It turned out Dean was accurate. Prosecutors demanded more tapes, and the case went to the Supreme Court.

On the morning of July 24, 1974, the high courtruled unanimously that Nixon had to turn over the subpoenaed tapes. That evening, the House Judiciary Committee began its televised deliberations on whether to recommend impeachment. Repeatedly Nixon’s defenders found themselves asking, as journalists summarized, “Where’s the smoking gun?” You cannot impeach the president without direct evidence of a serious crime. The impeachment articles passed with some Republican support. But the die-hard Nixon supporters had found the firm ground on which they would continue to fight, or at least so they thought.

But there was still that Supreme Court ruling. Nixon gingerly urged one of his lawyers to listen to the recording made on June 23, 1972, just six days after the arrests. Nixon’s staff quickly realized they had found direct evidence of obstruction of justice. In that conversation, Nixon’s chief of staff, H.R. Haldeman, told Nixon that the investigation had strayed “back into the problem area.” Nixon ordered Haldeman to have the CIA go to the FBI and tell the probers to “stay the hell out of this” since national security demanded an end to the investigation.

“Don’t lie to them to the extent to say there is no involvement,” Nixon told Haldeman to script the CIA. “But just say this is sort of a comedy of errors, bizarre, without getting into it, ‘the President believes that it is going to open the whole Bay of Pigs thing up again.’” (One of the break-in masterminds, E. Howard Hunt, had helped organize the ill-fated invasion of Cuba, and several of the burglars were anti-Castro Cuban emigres. Oliver Stone’s conspiratorial movie Nixon has that “Bay of Pigs” reference really refer to the Kennedy assassination.) After two years of pretending he wanted justice done, it turned out, Nixon had in fact ordered the cover up in its first hours.

The “smoking gun” tape shattered Nixon’s remaining support. I was a Watergate-obsessed 14-year-old visiting Washington, D.C., on a family vacation. We were sitting in the gallery of the House of Representatives when the tape was released. Something big was happening, it was clear, when the press room across the chamber exploded in activity—a scene that looked like something from The Front Page, as reporters ripped sheets of teletype copy, grabbed phones, tapped away on typewriters and generally made a commotion. We watched mesmerized. Then the House Judiciary Committee chairman, Peter Rodino, walked onto the House floor, triumphantly holding a sheet of paper above his head. Two dozen or so lawmakers crowded around him. In that pre-C-SPAN era, as Rodino read, the Democratic congressmen began to jump up and down and cheer. Rodino gave us tourists an okay sign in the gallery.

A few minutes later, we wandered into a small TV studio—where Nixon’s most vocal committee defender, Rep. Robert Wiggins of California, tearfully announced he would now vote for impeachment. They had found the “smoking gun.”

Rarely is there such obvious evidence of misconduct. But in the aftermath of Watergate, a series of rules and unwritten practices arose to prevent the kind of abuse of the investigative power that was at the heart of Nixon’s obstruction of justice. Administrations up to Barack Obama developed written policies. A group of former White House lawyers, United to Protect Democracy, set out the practices. Only a few key staff were allowed to talk about investigations at all. Only lawyers. No political operatives. Everyone knew that you had to treat investigations with great care. The president himself, notably, is one of the few officials allowed to have such contacts. It is assumed he would understand how explosive they could be, and would keep a safe distance.

I saw this firsthand when I worked for Bill Clinton as his chief speechwriter during a series of investigations that culminated in the Monica Lewinsky scandal. You didn’t need a West Wing pass to know the president loathed the FBI director, Louis Freeh. In public, Clinton bit his lip and focused in public on policy. In private, though, he seethed. Clinton had nominated Freeh, calling him a “law enforcement legend.” (Ouch. I drafted those words.) Quickly Freeh began to act in a way that convinced Clinton he was being partisan, reporting to Republicans on Capitol Hill and keeping information from the White House. Freeh urged Attorney General Janet Reno to appoint a special prosecutor to probe the 1996 Clinton reelection campaign and its supposed ties to China. Reno refused, and Freeh’s memo somehow leaked. Freeh complained that the President “victimized” his agency. “Friction Builds Between Freeh, Clinton, Threatening to Burn Them, and the FBI,” reported the Los Angeles Times.

Things only got worse during the impeachment year. I remember one meeting around the large table in the Cabinet Room in 1998. Jack Lew, then serving as budget director, explained that both the Secret Service and the FBI were competing for a pile of funds. “Well,” Clinton snorted, peering over his glasses, “one of those agencies has actually read the Constitution.”

Clinton would fume privately. But the very fact that he was under investigation made it necessary to treat the FBI with exquisite care. He simply could not fire Freeh, even though he no doubt would have relished the chance. It would have been inappropriate. It would also have provoked a firestorm.

George W. Bush understood the same thing. You simply had to respect the independence of a Justice Department investigation. When he had to decide whether to pardon a top associate, former vice presidential chief of staff Lewis “Scooter” Libby, Bush had his legal staff research the case and pore over trial transcripts, according to New York Times reporter Peter Baker. In the end, Bush did not issue the pardon. He knew he needed to protect himself and the presidency.

All of which brings us to today’s bombshell. Is this new allegation proof of criminal obstruction of justice? Let’s remember, for starters, that legal scholars generally believe a sitting president cannot be indicted. He can, however, be impeached. Firing Comey to turn off an investigation might be impeachable but not indictable. That’s the kind of abuse of power that checks-and-balances must prevent. But Comey’s new allegation is simpler, blunter, more brutal, and more plainly illegal.

The playwright Anton Chekhov (yes, Russian) famously said that if you put a gun on the table in the first act, it should be fired in the last act. The audience has barely settled into its seats. But in Donald Trump’s increasingly implausible drama, the gun is smoking.

Michael Waldman is president of the Brennan Center for Justice at NYU School of Law. He is the author of The Fight to Vote, published today in a new paperback edition.