On the evening of Saturday October 20, 1973, Federal Judge John Sirica sat in front of his television set watching FBI agents seal off the office of the Watergate special prosecutor Archibald Cox, who had just been fired at the command of the Nixon White House. The scene reminded him of a banana republic coup. “What the hell is this crowd doing?” he asked.

It’s far too early to say whether President Donald Trump’s firing of FBI Director James Comey will have the same dire consequences for his political future as Richard Nixon’s dismissal of Cox did for his. But not since that “Saturday Night Massacre” more than 40 years ago has a sitting president dared to fire an official in the middle of investigating potential misconduct by his own campaign. The risks of doing so are enormous.

“If President Trump thought the Russian hacking investigation would just go away, his decision today has ensured that it won’t,” said presidential historian Timothy Naftali. “It’s going to make getting rid of those allegations so much harder. There’s now a cloud of doubt.”

This is not the first time a president has fired the FBI director. Bill Clinton sacked William Sessions in 1993 amid allegations of alleged ethical misconduct — Sessions had used an FBI plane to visit his daughter, and had a home security system installed at government expense. But it is the first time a president has fired an FBI director who was probing possible misconduct by his own campaign aides or advisers — and on the recommendation of an attorney general who had already recused himself from the same investigation.

Trump muddied the issue further by saying he greatly appreciated Comey’s having informed him, “on three separate occasions,” that the president himself was not under investigation.

“It’s terrifying on so many levels,” said Michael Waldman, president of the Brennan Center for Justice at New York University Law School and a former chief speechwriter for Clinton. “This has every appearance of a cover-up, a possible act of obstruction of justice, just as much as Nixon firing Archibald Cox. That’s the only comparable historical precedent I can think of.”

Trump’s rationale for the firing — just days before Comey was to appear before the Senate intelligence committee — was not entirely clear. Last fall, on the eve of the election, Trump praised Comey’s re-opening of his investigation into Hillary Clinton’s use of a private email server as secretary of state. But a memo from Deputy Attorney General Rod J. Rosenstein laid out a bill of particulars faulting Comey’s handling of the Clinton investigation — both his initial decision to clear her, and his subsequent announcement that he was revisiting the issue.

In his own letter to Comey, Trump muddied the issue further by saying he greatly appreciated Comey’s having informed him, “on three separate occasions,” that the president himself was not under investigation, then adding that he concurred with the Department of Justice “that you are not able to effectively lead the bureau.”

That seemed to echo Nixon’s self-serving insistence to his Attorney General Elliot Richardson that Cox should be fired in “the national interest.” Richardson and his deputy, William Ruckelshaus, both declined to dismiss Cox before Solicitor General Robert Bork eventually carried out Nixon’s order. “Mr. President,” Richardson told Nixon, “it would seem we have a differing view of the national interest.”

“It’s very Nixonian, in its own way,” said Nixon’s former White House counsel John Dean, speaking of Trump’s move. “But that’s typical of the man. They’re obviously trying the get the bureau back under the Department of Justice’s control. But I don’t think it’ll affect the Russia investigation. You’ve got too many career people, and the counter-intelligence division — the cream of the crop — that will not take lightly to being messed with.”

Indeed, while the Democratic minority in Congress lacks subpoena power, there are any number of built-in institutional forces — including the FBI, the intelligence establishment and career prosecutors and lawyers scattered across the government that will all but assure the issue will not die — especially given how many of them will regard Trump’s motives as suspect on their face.

“The rationale is transparently absurd,” Waldman said. “Does anyone actually believe that Trump fired Comey because Comey was unfair to Hillary Clinton during the campaign?”

Forty-five years ago, Nixon’s resistance to cooperating with the legal processes of the Watergate investigation helped turn the tide against him — even among some Republicans.

A major question now is how Republicans on Capitol Hill will react to the firing. Already, some Democrats have called for the appointment of a special prosecutor to continue the investigation into whether anyone in the Trump campaign colluded with Russian interests to help sway the outcome of last November’s election. At least one top Republican, Senator Charles Grassley, chairman of the judiciary committee, supported the firing.

Forty-five years ago, Nixon’s resistance to cooperating with the legal processes of the Watergate investigation helped turn the tide against him — even among some Republicans — and his last shreds of support collapsed in the face of taped evidence of his own involvement in the cover-up of the break-in at Democratic National Committee headquarters. It is far from certain in the hyper-partisan climate of today’s Washington that Trump’s fellow Republicans will be anywhere near so inclined to break ranks, despite the extraordinary nature of the Comey firing.

Then, too, there is the issue of whom Trump might nominate to succeed Comey. Who would take the job in such a moment, and who could possibly be confirmed by the Senate? If Comey’s decision to make public statements about the Clinton case, even at the risk of influencing the outcome of the election, was a break with the non-partisan reputation that the FBI has worked hard to build up in the four decades since Watergate, Trump’s firing of the director landed as an even bigger blow to that status.

And Trump would do well to remember another lesson from the Nixon years. In the wake of Cox’s firing, the president’s men recommended the appointment of a new special prosecutor, a Texan who had been a “Democrat for Nixon.” His name was Leon Jaworski, and barely six weeks after his appointment, he secretly told the White House chief of staff Alexander Haig that Nixon had better hire a criminal lawyer. The reason: He had by then heard the tape of John Dean’s March 21, 1973 conversation with Nixon, in which the lawyer told the president that there was a “cancer” on the presidency and they discussed how to cover it up.