On the night of Saturday, October 20, 1973, Archibald Cox, the Watergate special prosecutor, asked that there be a consequence for actions that the President of the United States had just taken. “Whether ours shall continue to be a government of laws and not of men is now for Congress and ultimately the American people,” Cox said in a statement. Earlier that day, President Richard Nixon had ordered Cox fired and his office shuttered. In what became known as the “Saturday Night Massacre,” Attorney General Elliot Richardson, citing the rule of law, refused to carry out Nixon’s order, and resigned. So did the Deputy Attorney General, William D. Ruckelshaus. The Solicitor General, Robert H. Bork, the No. 3 official in the Justice Department, ultimately agreed to dismiss the special prosecutor.

The firing, though, proved to be a political debacle for the President. Cox became a national hero; Bork became a national villain. And, in a fatal political blow to Nixon, it accelerated the exposure of an astonishing scheme that brought elements of authoritarian rule to the United States, by which the President and his aides tried to use the F.B.I. and the I.R.S. to harass and spy on American citizens. In addition, a committee run by John Mitchell, the former Attorney General, had operatives, some of whom had worked for the F.B.I. and C.I.A., burglarize and bug the offices of the President’s political opponents, place false stories about them in the press, and spy on Democratic Presidential candidates’ staffers and family members. After news of these activities was first reported, Nixon told his aides to ask C.I.A. officials to help halt an investigation into them.

Hours after President Trump removed Attorney General Jeff Sessions from office, on Wednesday, Matthew Axelrod, a former senior Justice Department official, told me that, in the forty years since Watergate, Republican and Democratic Presidents alike have refrained from crossing a red line honoring the fact that federal law-enforcement officials should not be used for political purposes. “It’s a bedrock principle of the rule of law that decisions in criminal investigations be made based on the facts, not on politics, since Nixon,” Axelrod told me. “There has been a wall between the White House and the Justice Department in criminal investigations,” he added. “That has been critically important, so the investigations can be on the level, and the public can believe they are on the level.”

Axelrod, who was appointed by George W. Bush and served in the Justice Department for thirteen years during the Bush and Obama Administrations, said that Presidents do have the Justice Department carry out policies that reflect their political agenda, from antitrust investigations to civil-rights enforcement. But no President since Nixon has tried to influence criminal investigations. “White Houses have stayed out of investigations that involve criminal matters,” he told me. “All criminal matters, let alone criminals matters involving allies of the White House, or the President himself.”

Trump’s removal of Sessions crossed the red line. Sessions has faithfully carried out Trump’s agenda in multiple policy areas, from the travel ban to a sweeping immigration crackdown. The only area where he failed to implement the President’s wishes was in limiting Robert Mueller’s investigation of the President himself. Sessions’s replacement, the acting Attorney General, Matthew Whitaker, has publicly called for the Mueller investigation to be reined in and has suggested that the Justice Department defund it. Whitaker, a former federal prosecutor from Iowa who ran unsuccessfully for the U.S. Senate as a Republican, in 2014, has also said that there should be a criminal investigation of Hillary Clinton–something that Trump has repeatedly called for. (Sessions recused himself from overseeing the Mueller investigation and resisted calls to name a special counsel to investigate Clinton.) Whitaker is seen inside the Justice Department as politically motivated, loyal to Trump, and the “eyes and ears” of the White House.

Democrats warn that Whitaker will try to slowly “strangle” the Mueller investigation by cutting its funding or limiting it in other ways. “It is, in effect, a slow-motion Saturday Night Massacre,” Senator Richard Blumenthal, of Connecticut, a member of the Judiciary Committee, told MSNBC. “The reason that they are doing this in a slow-motion way is to avoid that uproar and uprising.” After Sessions’s firing, a handful of moderate Republicans, including Senators Susan Collins, of Maine, Jeff Flake, of Arizona, and the newly elected Mitt Romney, of Utah, called on Trump not to limit Mueller’s probe. But Flake is retiring, and the midterm elections shifted the Senate Republican caucus to the right and closer to the President. Most Republicans accepted Sessions’s removal and issued bland statements thanking the former Alabama senator for his service.

Axelrod declined to speculate on what actions Whitaker and Trump might take, but he was clear about recent history. “I worked at D.O.J. under both a Republican and a Democratic President. While some of the policies changed, the commitment to independence and the rule of law, to doing it the right way, never wavered,” he said. On Wednesday, for the first time in four decades, it did.