On October 20, 1973, Nixon crossed a fateful line by trying to rid himself of the independent special prosecutor who was investigating the Watergate scandal.
The prosecutor, Archibald Cox, had issued a subpoena for copies of secret tape recordings of Nixon's Oval Office conversations.
The tape recordings were key to proving Nixon's complicity in a sweeping cover-up of a break-in the year before at the Democratic National Committee offices at the Watergate, an office and residential complex on the Potomac.
Stung, Nixon reacted by ordering his attorney general, Elliot Richardson, to fire Cox.
Richardson refused and resigned in protest, so the president ordered the deputy attorney general, William Ruckelshaus, to do his bidding. But Ruckelshaus also resigned rather than carry out the order.
Both men had promised Congress that Cox would not be fired, except for just cause.
That left Cox's firing to the solicitor general, Robert Bork, who was next in line as acting attorney general.
Bork, who had made no promises to Congress, dismissed Cox.
The episode was a political and public relations disaster for Nixon.
For the first time, polls taken after Cox's firing showed the public was shifting in favor of Nixon's impeachment.
Cox was replaced by another special prosecutor, and Nixon eventually agreed to release transcripts of many of the tapes.
But the momentum toward impeachment became insurmountable, and Nixon resigned on August 8, 1974.
Nixon smiling?
Students of the era see striking similarities to Trump's firing of Comey, but also differences.
"In both, an angry and besieged president acted to remove an independent figure who was aggressively investigating people in the president's inner circle," Andrew Kent, a law professor at Fordham University in New York, told AFP.
One significant difference is that the president has full authority to fire the FBI director whereas, by law, Cox could only be fired "for cause."
Yet, only one other FBI director has been fired -- in 1993, then president Bill Clinton sacked William Sessions, and that was over relatively small-bore ethics infractions.
FBI directors traditionally have had enormous independence, and have guarded it jealously.
Democrats argue that Comey's firing underscores the need for a special counsel to investigate Russia's alleged meddling in the US election.
"Let’s remember that we face a looming constitutional crisis, very much like happened in 1973, the midnight massacre. This episode has very much the feel of that chapter in our history, one that we should not repeat," Senator Richard Blumenthal, a Democrat, said on CNN.
John Dean, Nixon's former White House counsel and a central protagonist of the Watergate scandal, on the other hand, says Trump's actions don't have the same feel at all.
"Archibald Cox was defying the president and taking his own course of action and making a decision that was very much placing Nixon in jeopardy," he said on PBS News Hour Tuesday.
The FBI is investigating whether Trump's campaign aides colluded with Russia to try to tilt the US election in his favor.
But the Trump administration cited "serious mistakes" in the director's handling of the investigation into Hillary Clinton's email as the reason for his dismissal.
"Somewhere Dick Nixon is smiling," Roger Stone, a Trump associate who once worked for Nixon, told The New York Times.
"Comey’s credibility was shot. The irony is that Trump watched him talk about bumbling the Hillary investigation, not the Russia investigation -- and decided it was time to get rid of him.”
