The idea that Donald Trump might fire—or try to fire—Special Counsel Robert Mueller has bubbled up enough times to seem possible, but still improbable. For one thing (as Reince Priebus and Sean Spicer, among others, can attest) press reports that this president might fire someone are frequently wrong. For another, it seemed that even Trump was prudent enough to avoid making the mistake that ended Richard Nixon’s presidency.

Yet Trump has a knack for making the wildly implausible suddenly imminent.  In the last 36 hours, the idea of Mueller being fired—and the political crisis it would likely set off—has become distinctly real. In an interview with The New York Times, Trump all but said he would fire Mueller if his investigation went into places Trump didn’t like. Since then, several reports have suggested that Trump’s defense strategy, as investigations probe deeper into his life and administration, is to attack Mueller and attempt to discredit him. Increasingly, the operative question seems not to be whether Trump will try to fire Mueller, but when he will do so and what will push him over the edge.

Firing Mueller would likely create a reprise of the October 1973 “Saturday Night Massacre,” in which Richard Nixon tried to fire Watergate special prosecutor Archibald Cox. When the attorney general and his deputy both refused and resigned, Nixon eventually got Solicitor General Robert Bork to do the deed. But a judge ruled the firing illegal, Cox was replaced by Leon Jaworski, and Nixon had to resign within a year.

If Trump did fire Mueller, it would be the third time in his tenure that Trump tried to get a law-enforcement official who was investigating him or his associates to close a case and, having failed, fired the official.

Preet Bharara, the U.S. attorney for the Southern District of New York, was, according to a Bloomberg report on Thursday, investigating financial dealings involving Trump, his son-in-law and senior adviser Jared Kushner, former campaign chairman Paul Manafort, and others. After winning the presidency, Trump told Bharara he intended to keep him in his job. Trump then worked to cultivate Bharara, placing repeated phone calls to him. Bharara refused to take the calls, saying they violated protocol. Trump then fired him, along with most other U.S. attorneys, in March. (Bloomberg reports Mueller has taken over the investigation Bharara started.)

Something similar happened with FBI Director James Comey. Trump invited Comey to dinner in January, where, according to Comey, Trump asked him for loyalty; Comey offered only “honest loyalty.” The following month, after National-Security Adviser Michael Flynn was forced to step down for lying to Vice President Pence about conversations with the Russian ambassador, Trump asked Comey to find a way to let Flynn ago, according to memos Comey wrote at the time. Comey did not, and in May, Trump fired him—citing the Russia probe as the reason in an interview with NBC News’s Lester Holt.

Mueller’s situation now looks eerily similar. The special counsel is known to be looking into Trump and his associates, both in their relations with Russia in the campaign and in their business dealings. Trump sent two of his lawyers to meet with Mueller, to ask him to wrap the investigation swiftly. Now, he has issued a warning to Mueller through the press. (His lawyers say they are cooperating with the investigation.) It’s difficult to believe that the special counsel will be intimidated. Mueller, himself a former FBI director, has a strong reputation for independence and doggedness. He might be even less susceptible to political pressure than Bharara and Comey, both of whom, while well-regarded for honesty, are sometimes accused of political ambition. (Mueller’s aversion to attention means it’s harder to know what’s going on inside his team, which doesn’t leak much.)

This places Trump and Mueller on a collision path. Either the president will have to fire the special counsel for doing exactly the same things that got Bharara and Comey axed, or he’ll have to sit and seethe as Mueller pokes into his taxes, his business, and who knows what else.

In mid-June, Chris Ruddy, a friend of Trump’s and the CEO of Newsmax, told PBS’s NewsHour that Trump was considering firing Mueller, on the basis that he had spoken to Mueller about the job of FBI director days before Deputy Attorney General Rod Rosenstein appointed him special counsel. The president felt that created a conflict of interest, but cooler heads in the White House seem to have convinced him firing Mueller was unwise.

Legal experts think Trump could fire Mueller in several ways. He could direct Rosenstein to do so, but Rosenstein would probably refuse unless there was a strong legal justification. Trump could also try to change the rules for firing, but that would also have to go through Rosenstein. Either path is fraught with likely firings or resignations at the Justice Department.

Yet in the eye-popping Times interview Wednesday, reporter Michael Schmidt asked, “If Mueller was looking at your finances and your family finances, unrelated to Russia—is that a red line?” Trump said, “I would say yeah. I would say yes … I think that’s a violation. Look, this is about Russia. So I think if he wants to go, my finances are extremely good, my company is an unbelievably successful company.” Trump wouldn’t actually commit to firing Mueller if he did, though: “I can’t, I can’t answer that question because I don’t think it’s going to happen.”

Since then, the flood. The Washington Post reports that Trump is seeking ways to box in Mueller’s probe and limit its scope, as well as exploring the limits of his power to pardon aides, or, potentially, himself. “They are actively compiling a list of Mueller’s alleged potential conflicts of interest, which they say could serve as a way to stymie his work, according to several of Trump’s legal advisers,” the paper adds. The New York Times had a similar report.

The Trump team seems to be targeting Mueller from two angles. The first is conflicts of interest. Trump seems to have little understanding of what constitutes a conflict; he has remained deeply entangled in his private business while serving as president, and accused multiple figures of conflicts of interest in his Times interview, even as he evinced no understanding of the conflict that forced Attorney General Jeff Sessions to recuse himself from Russia matters. But the Justice Department has explicit rules for what constitutes an improper conflict. It doesn’t appear that what the Trump team has come up with so far—Mueller’s conversation with Trump, or political donations by members of his team—would meet the standards in that policy.

The second tack is to try to prevent Mueller from moving into areas Trump doesn’t want him to explore. “The president’s making clear that the special counsel should not move outside the scope of the investigation,” White House spokeswoman Sarah Sanders said Thursday. Yet any argument that the investigation must stay within its own scope begs the question: Who is to determine what the scope of the investigation is, after all? Rosenstein’s letter appointing Mueller seems to offer the prosecutor a great deal of leeway, including authorizing “any matters that arose or may arise directly from the investigation.”

Take the Trump team’s warnings to Mueller to stick to Russia. The problem is that, as Trump surely knows, business doesn’t stop neatly at international borders. For example: Trump banks with Deutsche Bank, a German bank. Deutsche Bank works with Vnesheconombank, a state-owned Russian bank with whose chief executive Kushner had a questionable conversation in December. Or: Paul Manafort is reportedly being investigated for transactions through Cyprus, where Russian oligarch Dmitry Rybolovlev is chairman of the Bank of the Cyprus; Rybolovlev in 2015 bought a house in Florida from Trump for a huge profit. How does one draw a line between what is “Russian” and what is not?

While Mueller is not speaking to the press, various reports have emerged about the scope of his investigation, and they suggest that Mueller intends to follow each thread as far as he can. The historical precedent, as I have written before, is the Whitewater investigation into the Clintons. That inquiry didn’t end up finding wrongdoing in the 1970s real-estate deal that gave the scandal its name, but once a special prosecutor begins combing over someone’s affairs, he tends to find something. In Clinton’s case, the end game was impeachment in the Monica Lewinsky case, an affair that hadn’t even begun when the investigation opened.

Trump, who has made a career in business out of frequently bending or even simply breaking the rules, may have good reason to be concerned. The question is about what. The Post reports:

Trump has been fuming about the probe in recent weeks as he has been informed about the legal questions that he and his family could face. His primary frustration centers on why allegations that his campaign coordinated with Russia should spread into scrutinizing many years of Trump dealmaking. He has told aides he was especially disturbed after learning Mueller would be able to access several years of his tax returns.

Trump has famously refused to release his taxes, breaking a precedent that has endured since Watergate. During the campaign, he claimed he couldn’t release the taxes because he was under audit (he never proved that, and the IRS said there was no reason he couldn’t release the returns anyway), but since winning election, he has made clear he actually has no intention of releasing them.

The complaint about tax returns suggests two possible worries. One is that he thinks his returns will reveal improprieties or illegal behavior. The other is that Trump’s taxes will show that he is not worth as much as he claims he is, or that they will show that his debt dwarfs his assets. Being revealed to be in debt, or less rich than claimed, might be a strange reason to risk blowing up one’s presidency and by extension reputation and legacy. But Trump has both consistently exaggerated his wealth, attributing huge value to intangible things, and has fought bitterly when anyone has questioned his figures.

When journalist Tim O’Brien wrote that Trump was worth only $150 to $250 million, Trump sued him for libel in 2006, demanding $5 billion in damages. (That’s one way to build up net worth.) The suit didn’t go well. In a deposition for the case, Trump had to admit lying 30 times, and a judge dismissed the suit.

It is impossible to predict what might happen if Trump did fire Mueller. Republicans in Congress have shown relatively little interest in aggressively holding the president accountable. As McKay Coppins reported this week, many of them are dubious that Trump’s campaign colluded with Russia. But the collapse of the health-care bill combined with Trump’s threats against Mueller has aroused new ire among members. Senator Susan Collins of Maine told CNN, “It would be catastrophic if the president were to fire the special counsel.” Others expressed grave concerns, though not attaching their name.

Yet talk now and action later are two different things. On the one hand, plenty of Republicans have been critical of Trump but continue to mostly go with the flow. On the other, congressional Republicans were slow to turn on Nixon, too. In the event of a firing, James Fallows writes, holding Trump accountable would hinge on finding three Republican senators willing to buck the White House.

In some ways, Trump is already following in the steps of the 37th president. A friend of Donald Trump Jr. recently compared him to Nixon giving his famous 1952 Checkers speech, in which the then-vice presidential candidate defended himself against accusations of financial impropriety. That speech was a political triumph: It convinced Dwight Eisenhower to leave him on the ticket, Eisenhower won the presidency, and Nixon came back from the political dead, not for the last time. Biographer Jack Farrell notes that Nixon’s impetus for firing Cox was his fury that the special prosecutor had expanded the scope of his investigation past Watergate and into Nixon’s personal affairs. What is less remembered about the Checkers speech is that, as the Watergate investigation found, Nixon’s financial affairs really were dubious; he was wildly underpaying taxes. A politician can stave off the inevitable with public rhetoric and even firings for a time, but investigators often have the last word.