Robert Mueller’s investigation into Russian interference in the 2016 presidential election is following the facts and bearing fruit. Meanwhile, speculation abounds that President Trump will fire Mueller. Trump reportedly has already tried, backing off only after White House counsel Don McGahn threatened to resign rather than follow through.

If and when the president tries again—this time finding a more compliant White House official to agree to lower the boom—the fate of our democratic institutions hangs in the balance.

Protecting Mueller and his investigation—particularly at the very moment of its maximum vulnerability—could not be more crucial. History suggests a way forward.

On April 30, 1973, with evidence of White House complicity in the Watergate cover-up mounting, President Richard Nixon announced the resignation of several administration officials, including the attorney general. As his replacement, Nixon tapped his well-respected secretary of defense, Elliot Richardson. In a bid to answer his critics, the president also announced, fatefully, “I have instructed him that if he should consider it appropriate, he has the authority to name a special supervising prosecutor for matters arising out of the case.”

Unsatisfied, senators from both parties refused to confirm Richardson absent assurances that such a “special prosecutor” would be truly independent.

They prevailed. Richardson wrote a letter promising the special prosecutor “a truly unique level of independent authority within the Department of Justice” and that “[h]e will be subject to removal only by reason of extraordinary improprieties on his part.” Then Richardson named Archibald Cox as special prosecutor, and was confirmed as attorney general.

That July, when William Ruckelshaus was appointed to be deputy attorney general, the Senate Judiciary Committee, with extraordinary foresight, extracted the same promise: that Ruckelshaus would not fire Cox except for “extraordinary improprieties.”

Any student of American history knows what happened next. On Saturday, Oct. 20, 1973, Cox sought to compel the president to hand over his White House recordings. White House chief of staff Alexander Haig ordered Richardson to fire Cox. Richardson quoted his pledge to the Senate Judiciary Democrats in his letter of refusal—and resignation. Ruckelshaus, who then became acting attorney general, received the same order from Haig, before he too resigned.

The next person in the Justice Department’s chain of command was Solicitor General Robert Bork, who wasn’t asked any questions about special prosecutors during his confirmation hearings the previous January.

And so Bork complied with the president’s order and the “Saturday Night Massacre” went forward.

A tsunami of public outrage followed. Nearly half a million telegrams of protest were sent to federal officials within the next two weeks. Thanks to that outpouring, the investigation of President Nixon survived with a new special prosecutor, whose position was written into law by Congress.

In refusing to fire Cox, Richardson and Ruckelshaus proved themselves to be men of their word. Had they not been asked to make that promise, they may have followed the orders of the White House. In fact, Richardson indicated at the time that had he not committed to Congress that he would protect Cox, he would have felt duty bound to fire him when asked. The Saturday Night Massacre that claimed several high-profile administration officials might have merely been the silent assassination of the special prosecutor—and the “Watergate Special Prosecution Force” may not have survived.

Today, the partisans controlling Congress cannot be counted on to preserve Mueller’s independence. Some Republican members—who apparently would rather place American democracy itself in peril than let anyone in Donald Trump’s White House face the music—have gone so far as to call for the criminal prosecution of Justice Department officials supervising the investigation.

That is why top officials in the Justice Department’s line of succession must provide what Richardson and Ruckelshaus did in 1973: a written and public commitment of noninterference in the special counsel’s investigation into Russian meddling in the 2016 election, any possible collusion with the Russians by the Trump campaign, and related efforts to obstruct justice.

Nine Senate Judiciary Committee members have written these officials seeking such a commitment. These officials have taken an oath to uphold the rule of law and protect the Constitution. This must include refusing to take part in any effort to shield the American public from the truth and any attempt by the president to obstruct justice.

We expect that those charged with the solemn responsibility of protecting the integrity of American institutions will pledge publicly that the investigation can proceed without interference. Nothing less than America’s liberty depends on it.