Unless something changes again in the mercurial mind of Donald Trump, it looks like Rod Rosenstein will survive the day. He survived last Friday, Sept. 21, when the New York Times published a report that he had raised the possibility of wearing a wire in the White House and had discussed invoking the 25th Amendment against the president. He survived Monday, Sept. 24, amidst warring press reports about whether he would resign or be fired. And then he was scheduled to meet with President Trump on Thursday, Sept. 27—the same Thursday that Supreme Court nominee Brett Kavanaugh will face the Senate Judiciary Committee—raising expectations once again that the deputy attorney general would be making his exit. But Wednesday evening, Trump appeared to be having second thoughts.

In a press conference earlier that day, Trump addressed his plans for Rosenstein, suggesting the Thursday meeting might not even take place:

REPORTER: Mr. President, you have another meeting tomorrow with Rod Rosenstein. TRUMP: Yes. REPORTER: Are you planning to fire Rod? TRUMP: I’m talking to him. We’ve had a good talk. He said he never said it. He said he doesn’t believe it. He said he has a lot of respect for me. And he was very nice and we’ll see. And he’s a member of the Trump administration. In that sense. It’s the Justice Department. I would certainly prefer not doing that. There was no collusion. There was no obstruction. I mean, unless you call obstruction the fact that I fight back. I fight back. I really fight back. If you call that obstruction, that’s fine. But there’s no obstruction. There’s no collusion. I’m going to meet with him tomorrow. I may call Rod tonight or tomorrow and ask for a little bit of a delay to the meeting, because I don’t want to do anything that gets in the way of this very important Supreme Court pick. So I don’t want it competing and hurting the decision. One way or the other decision. Again, I want to hear what she has to say. But I don’t want to—so I may delay that. I’m going to see. I don’t want to do anything that’s going to conflict with that. But my preference would be to keep him. And to let him finish up. You know, I call it a witch hunt. And it is a witch hunt.

And NPR reported this morning that there is no meeting with Rosenstein on the president’s calendar today.

Making fun of a clownish figure in Twelfth Night, Shakespeare wrote that “Some are born great. Some achieve greatness. And some have greatness thrust upon them.”

Greatness in not the right word for Rosenstein. Whatever happens today, he will never measure up to Attorney General Elliot Richardson, who was fired by President Richard Nixon for his refusal to dismiss Watergate Special Prosecutor Archibald Cox during the Saturday Night Massacre. But for whatever reason, and by whatever means, Rosenstein has grown immeasurably in the role he has played. He has become an essential bulwark in the defense of American institutions of the rule of law. He has held a deeply important line. And his departure would thus create a dangerous hole.

“Rod is a survivor,” James Comey once said privately of Rosenstein, not meaning it as a compliment. At this point, however, survival is a great virtue on Rosenstein’s part. Every day he stays put, Special Counsel Robert Mueller gets to do his job absent interference. Every day Rosenstein doesn’t resign buys the special counsel time. Every day Trump doesn’t fire him allows something roughly approximating the regular order to prevail at much of the Justice Department. The day Rosenstein is actually forced out will be a very bad day.

Yes, he is a complicated figure. His current service in government began with the original sin of fronting for the Comey firing. And it actually says a lot about the collapse of the unitary executive under Trump that Rosenstein is still in office after the Times’s revelations. In a normal presidency, a story like the one the Times ran on Friday about Rosenstein would make continued service impossible, and the time frame for exit would be minutes or hours, not days or weeks. Then again, in a normal administration, Defense Secretary James Mattis would not have survived the publication of Bob Woodward’s book “Fear"—which reported that Mattis defied a presidential order for a military strike against Syria. And in a normal administration, cabinet officers would not serially contradict the president in public. The secretary of state would not survive an hour after saying that, “the president speaks for himself.” In a normal administration, White House aides don’t purloin documents from the president’s desk to prevent him from signing them.

At this point, the role Rosenstein plays on a day-to-day basis far eclipses in importance any anger or contempt he earlier earned or any eyebrows that his recently-reported conduct may raise. Rosenstein may not achieve greatness, but he has had a crucial role thrust upon him. And unlike a lot of people in Washington, he has risen to the occasion. He has done so under incredibly difficult circumstances. The importance of the role he has played must not be underestimated.

It is not just that Rosenstein appointed Mueller. He could have appointed the special counsel and then washed his hands of the matter, leaving Mueller to fend off congressional and presidential pressures on his own. He didn’t. He has assiduously defended the Mueller investigation, even as the president has decried it as a “witch hunt,” even as congressional committees have pressed for unprecedented document releases by way of discrediting it, even as an entire media ecosystem has risen up to attack it, even as that ecosystem has painted him as a conspirator against the president.

One way of understanding the role that Rosenstein has played is to think about who might take up that role in his absence. That’s a wildcard. In the Justice Department’s chain of command, Rosenstein’s departure would mean that Noel Francisco—the solicitor general—would become the acting attorney general for purposes of matters from which Attorney General Jeff Sessions is recused. But Francisco may have a similar issue of his own: he has recused himself from other matters involving his former law firm, Jones Day, whose representation of the Trump campaign could prompt Francisco’s recusal from the Russia probe as well.

While it’s unclear who would supervise Mueller in the short term, in the longer term, the role would shift to whomever the president appoints and the Senate confirms to replace either Rosenstein or Sessions. That puts an enormous premium on an opaque staffing matter—in an immediate sense—and also on the Senate’s ability to ensure that the people nominated to take the helm at the Justice Department will continue to hold the line in Rosenstein’s absence. Given the president’s open belief that the role of the Justice Department’s leadership is to protect him and go after his enemies, and given the Senate Judiciary Committee’s lackluster commitment to restraining the president in his interactions with law enforcement, we can be confident of neither.

What happens if the president is ultimately able to install a loyalist to oversee the special counsel investigation? What kind of mischief could be made by someone hostile to the Mueller probe and seeking to protect the president and his allies?

The short answer is a whole lot.

In the extreme case, of course, that person could simply fire Mueller. Such a move would create a political firestorm, but a compliant acting attorney general who fired Mueller absent a specific direction from the president could generate a lot of cover for the White House.

Short of a firing, a Rosenstein successor inclined to stymie the Russia probe would have a bunch of lesser tools—the simplest of which might not require interacting with the special counsel at all. A new acting attorney general could decide to participate with the efforts of congressional Republicans to harm the probe, a course the president has already publicly endorsed. Rosenstein has so far acted as a bulwark in refusing the hand over particular documents, though he has capitulated on other demands. Malicious actors in the House of Representatives—for example, intelligence committee chairman Devin Nunes and Representatives Mark Meadows and Jim Jordan—know that forcing disclosures or facilitating leaks of information about an ongoing investigation is a great way to muck things up. To the extent they have not fully succeeded in that effort, it’s partly thanks to Rod Rosenstein. A replacement could change that.

A new acting attorney general could also inflict damage through the exercise of oversight powers. Rosenstein did not rely on the special counsel regulations themselves, but rather a different set of authorities, to actually appoint Mueller. But in his order, he expressly applied the special counsel rules set forth at 28 USC 600.4-600.10. Those are the rules that broadly define the special counsel’s general powers and, importantly, scope the relationship between the special counsel and the attorney general (or acting attorney general, as the case may be).

These rules are explicit that the acting attorney general is still ultimately responsible for the investigation. A new acting attorney general might have a tough time altering the original jurisdiction granted by Rosenstein, but the special counsel is required to request from the attorney general any additional grants of jurisdiction he might deem necessary. The attorney general is then permitted to grant the request or assign the matter “elsewhere.” Denying additional jurisdictional requests would be a good way to limit the investigation.

So far, Mueller has been careful to spin off aspects of the probe that aren’t within the core of his jurisdiction—such as the Michael Cohen matter, which he referred to the U.S. Attorney’s Office for the Southern District of New York—so this might not be an effective lever against the special counsel’s office. On the other hand, you never know. In additional to new requests for jurisdiction, the special counsel usually consults with the acting attorney general on what falls within his original jurisdiction in the first instance. If a new acting attorney general decided to take a particularly narrow reading of the jurisdiction in Rosenstein’s original order, he or she could substantially limit the inquiry.

The biggest and most frequent opportunity to obstruct the investigation comes in the broader consultation requirement. Throughout the investigation, the regulation requires that the special counsel “shall consult with appropriate offices within the Department for guidance with respect to established practices, policies and procedures of the Department, including ethics and security regulations and procedures.” The special counsel is also required to notify the acting attorney general “of events in the course of the investigation in conformity with the Departmental Guidelines with respect to Urgent reports.” In practice, that means Mueller has to tell the acting attorney general about any “major developments,” like filing criminal charges, in advance.

Such information-sharing alone could be a problem if sensitive law enforcement information improperly made its way to the White House—much less to subjects of the investigation. Beyond such obviously improper conduct, however, an acting attorney general could determine under the rules that a proposed action should not be pursued at all. The applicable regulation reads:

The Special Counsel shall not be subject to the day-to-day supervision of any official of the Department. However, the Attorney General may request that the Special Counsel provide an explanation for any investigative or prosecutorial step, and may after review conclude that the action is so inappropriate or unwarranted under established Departmental practices that it should not be pursued. In conducting that review, the Attorney General will give great weight to the views of the Special Counsel.

Note that it isn’t entirely clear whether that kind of determination would be binding on Mueller—“should not be pursued” is something short of “shall not be pursued.” But Mueller’s own team acknowledged in court that if the special counsel were to disobey an order from the acting attorney general, that would constitute “good cause” for firing him. So Mueller can only defy orders until the point the acting attorney general is prepared to fire him over it.

One mild protection for Mueller is that if the acting attorney general ever determines that an action is so inappropriate or unwarranted as to not be pursued, he or she has to report that to Congress along with the justification. But the regulations don’t require that report until the “conclusion of the Special Counsel’s investigation.”

Finally, and perhaps most importantly, the regulations designate the acting attorney general as the person to decide, at the conclusion of the investigation, whether any report is made public or shared with Congress. Numerous press accounts, as well as the president’s lawyers, have suggested that Mueller is preparing a report of some kind on the obstruction of justice components of his investigation. Ask yourself this: Are you more comfortable with Rosenstein or with some unknown replacement deciding whether and how to handle that document?

The relationship between the acting attorney general and the special counsel is ultimately one of comity, as both parties try to achieve the delicate balance between independence and accountability. That means, however, that a lot of Mueller’s independence is a question of the acting attorney general’s discretion. Mueller has discretion of his own to exercise as well. At least in the first instance, the special counsel decides what matters he is required to consult with the acting attorney general about, what requires urgent notification within the guidelines, and, of course, he gets to decide if he is willing to follow an order. So far, under Rosenstein, we have seen no evidence of friction between the two.

There is one prudential limitation Mueller has observed thus far and that he might reconsider if he feels his investigation is being hampered: His public silence. There is no rule that requires the special counsel to keep his mouth shut. That means he doesn’t have to rely on congressional notifications; he can hold a press conference and tell the American people if he believes the acting attorney general is obstructing his work. Mueller’s silence is itself a vote of confidence in Rosenstein.

Because of the bizarre manner in which Trump has flirted with Rosenstein’s dismissal—the on-again-off-again plans for the deputy attorney general’s firing, the rage tweeting, and the repeated public signaling—Mueller has had a lot of time to prepare. Anyone in his shoes would be foolish not to have thought through the steps he means to make to protect his investigation. Without knowing the precise state of the investigation, it is useless to speculate what those steps might be, and whether any would take place immediately or whether they might wait until some Rosenstein replacement endeavored to engage the special counsel in a fashion differently than Rosenstein has done. But it’s fair to say that Mueller will not be caught flat-footed by whatever happens.

That’s a good thing. And assuming Rosenstein survives the day, that will be a good thing, too. Every day this crisis can averted, slowed, or mitigated is an extra day that Mueller has to do his job. It’s a day closer to electoral accountability. It’s a day without a conflagration this country does not need.