To many establishment Republicans, the prospect that President Donald Trump would fire special counsel Robert Mueller is unimaginable.

“I cannot possibly imagine the president terminating Bob Mueller,” Sen. Bob Corker (R-TN) said in June. “It just cannot happen.”

It’s time to start imagining it.

Mueller’s probe is looking ever more dangerous to Trump. In October two of his former campaign aides were indicted, and a third pleaded guilty on false statement charges. Then, on December 1, his former National Security Adviser Michael Flynn pleaded guilty to a false statements charge, and made a deal cooperate with Mueller’s team.

Even before all this, the president repeatedly complained about the investigation both in public and in private, and some of his allies in conservative media started preparing a public justification for the firing of the special counsel.

Plus, there are historical precedents. In 1973, Richard Nixon fired his way through the Justice Department until he could fire the special prosecutor investigating Watergate. And in May 2017, the sitting US president — a guy named Donald Trump — fired the FBI director who was leading an investigation into his own campaign.

So this could really happen. But as we consider the possibility that Trump will move on Mueller, there are a couple of things to keep in mind.

First, it wouldn’t be as simple as firing one person. It seems that to get rid of Mueller (without a finding of genuine misconduct), Trump would also have to get rid of Deputy Attorney General Rod Rosenstein — and perhaps several more Justice Department officials, until he finds one willing to carry out his order. Meaning it would get very messy.

Second, the past precedents of the Saturday Night Massacre and the Comey firing both ended up failing to squelch the respective investigations at play. Both provoked a backlash from the public and the political system, and eventually the existing investigation ended up continuing under new leadership.

So getting rid of Mueller would be enormously costly to Trump, and could well fail to end his legal problems. This is likely why White House aides are currently advising against it.

But it’s entirely possible that, whether out of fear of what Mueller will turn up or simple annoyance at the investigation itself, Trump will pull the trigger anyway. And if he does so, the political system will be thrown into a major crisis.

Firing Mueller would be more complicated than firing Comey

Trump clearly had the authority to fire FBI Director James Comey, solely on his own say-so. But with special counsel Robert Mueller, things aren’t so simple.

A Justice Department special counsel is essentially intended to be an outsider who would feel less pressure to carry water for his or her bosses in government, and would be more willing to expose wrongdoing from people in power — someone like Mueller, a well-regarded lawyer and former FBI director near the end of his career.

The special counsel procedures were established by a Justice Department regulation, and the authors of this regulation tried to make it deliberately difficult for the special counsel to be fired. They wrote that only the attorney general can fire him or her, and that even then, there would have to be a good reason — that it could only be done for “misconduct, dereliction of duty, incapacity, conflict of interest, or for other good cause.”

In Mueller’s case, there’s another twist here. Due to Attorney General Jeff Sessions’s recusal from all campaign-related investigations, Mueller is not reporting to Sessions, but instead to the deputy attorney general, Rod Rosenstein. So according to the regulation, it would have to be Rosenstein who fires Mueller.

Rosenstein confirmed in sworn congressional testimony back in June that he interpreted the special counsel regulation to mean that he is the only person in government with authority over Mueller’s hiring and potential firing — that is, not Trump and not Sessions. Furthermore, he said that he would only fire Mueller for “good cause,” and would refuse to carry out an order to fire him if good cause were not established.

So it would appear that Trump is stuck with Mueller, since only Rosenstein can fire him, and Rosenstein has sworn not to do it without a really good reason.

Yet if Trump wanted Mueller gone badly enough, he could use one of a few possible workarounds. For one, if Jeff Sessions resigned or was fired, Trump’s appointee to replace him as attorney general could become Mueller’s new boss and carry out the firing. Alternatively, since the special counsel’s authority is from regulation and not law, the Trump administration could theoretically try to roll back that regulation somehow — though that could end up in court.

If Trump wanted to get Mueller out quickly, though, the way to do that would be to fire his supervisor, Deputy Attorney General Rosenstein. Then he’d order the Justice official elevated as Rosenstein’s temporary replacement — Associate Attorney General Rachel Brand — to fire Mueller. If Brand also refused, Trump would fire her, repeating the process until he finds someone willing to carry out his order.

We know this can work, because it’s happened before: It’s what President Richard Nixon did during the Saturday Night Massacre.

The Saturday Night Massacre, explained

By October 20, 1973, Justice Department special prosecutor Archibald Cox had been investigating the Watergate hotel break-ins and related crimes — crimes that appeared to implicate many of the president’s top aides — for months. He’d taken an aggressive approach, too, waging court fights to try to force Nixon to turn over White House recordings of his conversations with his aides.

Knowing the tapes were deeply incriminating, Nixon searched for a way out. He ordered Cox to stop trying to get the tapes, but Cox refused to obey. Then White House aides leaked that the president was considering firing Cox. But Cox held a press conference in which he asserted that, according to his interpretation of the law, only the attorney general could fire him. He said that if Nixon tried to fire him, he’d fight the decision in court.

So Nixon ordered the attorney general, Elliot Richardson, to fire Cox. But Richardson refused, and resigned rather than carry out Nixon’s order.

After that, Deputy Attorney General William Ruckelshaus was in charge of the Justice Department, and Nixon gave him the same order to oust Cox. He too refused, and resigned.

Now, according to the Justice Department’s line of succession, Solicitor General Robert Bork was in charge. And finally he agreed to carry out Nixon’s instruction to fire Cox, and abolished the office of the special prosecutor to boot.

A tremendous public uproar ensued. “The term ‘Constitutional crisis’ has been overused in the course of the year. … But now this may be the real thing,” journalist Elizabeth Drew wrote at the time, in what became her excellent book Washington Journal.

Past attempts to squelch investigations have often failed

The Saturday Night Massacre wasn’t the end of the story, though. In the ensuing 11 days, Democrats excoriated Nixon, and Republicans in Congress began turning against him too. The backlash and pressure were too much to bear.

So on November 1, Nixon’s administration announced they’d appoint a new special prosecutor, Leon Jaworski. Furthermore, Bork said the president had agreed not to fire Jaworski unless he had bipartisan support among congressional leaders. The investigation would continue and, eventually, lead to Nixon’s downfall.

An important lesson, then, is that extreme measures like this to squelch an investigation can often provoke a similarly massive backlash from the public and other actors in the political system. This is a lesson Trump learned earlier this year, when he used a thin pretext to fire Comey and, after eight days of uproar, ended up with Mueller appointed to take over the Russia investigation as special counsel.

But keep in mind that none of these outcomes were guaranteed. Drew’s book chronicles the feeling of uncertainty and even terror that many felt during those 11 days between Cox’s firing and Jaworski’s appointment — particularly when Nixon declared a worldwide military alert for vaguely explained reasons.

Similarly, it was Rosenstein who made the choice to appoint Mueller. A more hackish appointee in his position, or someone who was less concerned with his reputation in the legal community, might not have done the same — or might have appointed a special counsel who was far less aggressive and experienced than Mueller. And if Rosenstein had not acted, it’s unclear what, if anything, Republicans in Congress would have done.

Mueller’s firing would mean a crisis over the rule of law

The big picture is that Mueller’s firing would have seismic implications.

Though Trump would surely come up with some pretext (just as he initially did with Comey), the message would be loud and clear to all: The president is willing to fire anyone who dares investigate him or people close to him. It would mean no less than a crisis over the rule of law in the United States.

Now, the FBI investigation itself would likely continue (unless Trump installs someone who affirmatively shuts it down). But without Mueller at its head, it’s unclear how aggressive it would be, or who would be running it.

So the ball would then be in congressional Republicans’ court to decide whether firing Mueller was a step too far for President Trump.

Partly in fear of angering their own voters, and partly because they hope to win policy victories in a rare moment of unified control of Washington, most elected Republicans have generally either ignored Trump’s most controversial statements and actions this year, offered them only the mildest of rebukes, or outright backed them.

Yet several have said that firing Mueller would be going too far. “If he fired Bob Mueller, I think you’d see a tremendous backlash, response from both Democrats but also House Republicans,” Rep. Mike McCaul (R-TX) said in July. It “could be the beginning of the end of the Trump presidency,” Sen. Lindsey Graham (R-SC) warned that same month.

But lately, pro-Trump conservative media outlets and commentators have been trying to lay the groundwork for Mueller’s potential ouster. For instance, the Wall Street Journal editorial board wrote last week that Mueller should resign because he couldn’t be trusted to impartially investigate the conduct of his old colleague James Comey and the FBI itself. Fox News, too, has been increasingly critical of Mueller of late.

So when it comes down to it, would Republicans prove willing to defy their president — a president who remains extremely popular among GOP primary voters — to force Congress into action that would ensure the Russia scandal is investigated and the rule of law is protected in America?

Or would they choose to become knowing participants in what would then unmistakably be a presidential cover-up of historic proportions?

It’s impossible to know right now. But what we do know is that the health of American democracy would depend on whether congressional Republicans decided to buck a president from their own party.