President Donald Trump’s extraordinary decision to fire the FBI director James Comey is a stunning moment in American politics, one that has historians reaching back decades for any parallels.

Comey’s termination, which the president says was a recommendation from the Justice Department, comes as the FBI continues to investigate allegations that people involved in Trump’s presidential campaign had undisclosed ties to Russia. Comey was supervising that investigation.

“This is bad,” said Julian Zelizer, a history professor at Princeton. “There’s no way to see it other than: this is a high point of tension and it is in some ways an act of presidential imperialism against the point man in this investigation.”

“I don’t revere James Comey as an arbiter of neutral justice,” said David Greenberg, a professor of history at Rutgers, “But the thought of having the president’s own appointee overseeing that is just tremendously worrisome. I don’t see how you can’t conclude that a coverup is likely to be in the works. I don’t suppose how anyone could come to any other conclusion.”

The details are unlike anything that’s happened in American history—it is only the second time an FBI director has been fired—but the contours of the firing have parallels. One name that keeps coming up: Richard Nixon.

“This is Nixon all over again,” Zelizer told me. “Lots of other presidents had tension with the FBI—that’s Lyndon Johnson or John F. Kennedy. Even Clinton had tension. But this is more Nixonian.”

Nixonian, he and others said, because of the brazenness of a president firing the FBI director at a time when the president’s colleagues are under investigation by the FBI. “And the irony is this comes after the FBI director played a big role in making him win,” Zelizer said, referring to Comey’s decision to make public a re-examination of Hillary Clinton’s handling of a private email server in the final weeks of the presidential election.

The only other time an FBI director has been dismissed previously is when Clinton’s husband, the former president Bill Clinton, fired FBI Director William Sessions in 1993 amid an ethics firestorm. Sessions had refused to resign despite the Justice Department’s determination that he’d abused his office.

At the time, Clinton said that it was clear Sessions “can no longer effectively lead the bureau and law enforcement community,” a statement that Trump echoed in a letter to Comey, telling him, “you are not able to effectively lead the bureau.”

Other U.S. presidents have often clashed with the FBI—J. Edgar Hoover was infamous among presidents and the public—yet many chief executives have favored a grin-and-bear-it approach. Bill Clinton had moments of heightened tension with Louis Freeh, the FBI director he appointed after Sessions, for example, but tensions never escalated as publicly as those between Trump  and Comey.

“The first 50 years or so of the FBI was all J. Edgar Hoover, and there was a very special relationship between him and all these presidents who were in a way quite deferential to him,” Greenberg said. “There were always conflicts and tensions. They sometimes had to rein him in and he sometimes pushed them around. So this is striking to see, post-Nixon, it really is a crisis.”

More than Hoover, Comey’s firing evokes the dismissal of Archibald Cox, the independent special prosecutor whom Nixon fired during the Watergate scandal. That firing, nicknamed the Saturday Night Massacre, didn’t undermine the investigation the way Nixon may have hoped.  

“There were concerns when Nixon fired Archibald Cox that his successor, Leon Jaworski, would not be as tough,” Greenberg said. “That turned out to be wrong. Jaworski was independent. He followed the evidence and he ended up being an effective special prosecutor. There is an external reality that can’t always be bent to partisan desires.”