In the last several days, President Donald Trump’s lawyers have floated what appears to be a trial balloon, suggesting that despite Trump’s bold proclamation that he welcomed the opportunity to testify to special counsel Robert Mueller’s team under oath, he might ultimately do a U-turn and refuse to be interviewed at all, under any conditions. If he takes this step, then Mueller could subpoena him to the grand jury, at which point Trump could avoid testifying only by pleading the Fifth Amendment, thereby asserting his right not to incriminate himself.

What then? Would Mueller be blocked? Maybe, but not necessarily. Mueller would have one last move he might make: he could decide to immunize Trump and thereby force him to testify. This step would be risky and in some ways unorthodox, but in the context of an investigation where little is ordinary, it is one that Mueller will have to consider seriously.

Already it reflects the extraordinary dynamics at play in this investigation that the question of Trump taking the Fifth has to be considered at all. Ordinarily, it would be political suicide for a President to take the Fifth. But as others have written, Trump and his allies have been laying the groundwork for a dramatic act to block the Mueller investigation, seeking to discredit Mueller as “biased” and the investigation as a “witch hunt.” Noah Feldman suggests that as a result, Trump just might be able to take the Fifth and get away with it. Possibly true, though political history is littered with the bodies of those who have overplayed their hand. Trump himself has proclaimed that if you are innocent you don’t take the Fifth, and even his core supporters might begin to wonder what he is hiding if he were to take this step himself.

If Trump does take the Fifth, Mueller could still force him to testify, by granting him “use immunity,” which is expressly allowed by federal statute. This form of immunity involves a commitment that Trump’s testimony would never be used against him in any way in a subsequent criminal proceeding. In theory, Trump could still be prosecuted for any past crimes, including those he testified about, but the reality is that because the bar to show that no “use” was made of a person’s immunized testimony is so high, prosecutors typically grant use immunity only to those persons they’ve decided not to prosecute anyhow. When prosecutors seek to prosecute someone who has testified pursuant to a grant of use immunity, they bear the burden of demonstrating not only that no investigator or prosecutor made use of the immunized testimony, but also that no witness was influenced by it. As was learned the hard way in the Oliver North prosecution, that can be a difficult burden to meet, particularly if the immunized testimony becomes public.

What is the process for getting an immunity order? The U.S Attorney’s Manual requires that prosecutors seeking to grant use immunity to a witness must obtain the approval of the Criminal Division of the Department of Justice (and if subsequently they want to prosecute someone who has testified under a grant of use immunity, they must obtain the permission of the Attorney General himself or herself). Grants of immunity are issued by a judge, on application of the government. Perhaps most importantly for Trump and his lawyers: a grant of use immunity does not cover the commission of perjury during the testimony itself. Therefore, to the extent Trump’s lawyers are counseling him not to testify because they are afraid he will lie, that danger would still exist if he were compelled to testify pursuant to an immunity order.

The U.S. Attorney’s Manual further lays out the factors that prosecutors should consider when deciding whether to grant immunity:

9-23.210 – Decision to Request Immunity—The Public Interest Section 6003(b) of Title 18, United States Code, authorizes a United States Attorney to request immunity when, in his/her judgment, the testimony or other information that is expected to be obtained from the witness “may be necessary to the public interest.” Some of the factors that should be weighed in making this judgment include: The importance of the investigation or prosecution to effective enforcement of the criminal laws; The value of the person’s testimony or information to the investigation or prosecution; The likelihood of prompt and full compliance with a compulsion order, and the effectiveness of available sanctions if there is no such compliance; The person’s relative culpability in connection with the offense or offenses being investigated or prosecuted, and his or her criminal history; The possibility of successfully prosecuting the person prior to compelling his or her testimony; The likelihood of adverse collateral consequences to the person if he or she testifies under a compulsion order. These factors are not intended to be all-inclusive or to require a particular decision in a particular case. They are, however, representative of the kinds of factors that should be considered when deciding whether to seek immunity.

Ordinarily, these factors result in a relatively straightforward calculation: in cases where prosecutors conclude they need evidence from “insiders” to crack open an investigation, they will immunize little fish to get to the bigger fish if and only if they believe that they will obtain reliable evidence and there is no other alternative. On this approach, it seems clear that Mueller’s team would never consider immunizing Trump. He is the top potential target in the Russia collusion investigation as well as the obstruction of justice investigation. What’s more, it is difficult to imagine the prosecutors ever relying on Trump’s testimony to prosecute another person. Trump is too easily impeachable as a witness and it is usually bad practice to prosecute down, that is to use the testimony of superiors to prosecute subordinates.

So, is that the end of the discussion? Well, maybe not. Consider at least three factors that make this investigation different from your typical federal probe. First, it is difficult to imagine that the result of Mueller’s investigation will ever be the criminal prosecution of Trump. By most accounts, Mueller is likely bound by the Department of Justice’s current position that a sitting President cannot be indicted, and even after Trump leaves office (after impeachment, resignation, or the completion of his term), it is hard to imagine criminal prosecution following as a result of Mueller’s investigation. Therefore, in this case, if prosecutors granted Trump use immunity, thereby complicating or even foreclosing a future criminal prosecution, they might not be giving up very much at all.

Second, while Trump will not likely face criminal prosecution, impeachment is another matter—and the Fifth Amendment can’t protect the President if Congress goes down that path. Depending on what Mueller’s investigation ultimately discloses, Congress may have to consider whether impeachment is warranted. And if they did take this step, members of Congress would not be foreclosed from considering Trump’s immunized testimony. An immunity order prohibits the use of testimony in future federal criminal prosecutions of the person providing the testimony, but does not foreclose use in any other type of a proceeding, including impeachment. That means that Mueller would not need to worry that by granting immunity to Trump he was in any way compromising the most likely form of accountability because the immunity order would have no bearing on any future impeachment proceedings. If anything, the immunity route might bolster the chances for such a proceeding because Trump’s refusal to testify would raise suspicions that he had something to hide, the grant of immunity would leave him with no valid excuse not to testify fully, and he would have to do so under the threat of perjury. It would then be left to those elected officials to decide what to do with the information.

Third, as others have written, Mueller’s investigation has both a criminal and a counterintelligence purpose. Particularly if Mueller believes that the criminal side of the investigation does not warrant criminal charges against Trump, he might decide that his counterintelligence objectives require hearing from Trump to fill in the whole picture. The broader public interest may be filled by having the President answer Mueller’s questions for the record. The grant of immunity would not limit in any way the use of Trump’s testimony for those purposes.

These factors potentially change the calculus a lot, and could push Mueller toward serious consideration of seeking an immunity order if Trump takes the Fifth. Ultimately, however, I suspect his decision will tilt against going down this road. Mueller is not an impeachment investigator, and therefore he cannot, and should not, take steps with the sole or principal purpose of bolstering that possible outcome. His investigative decisions either have to have a criminal purpose or a counterintelligence one. On the criminal side, immunizing Trump serves no real benefit because it would not advance a case against Trump or anyone else, and granting immunity to the top potential target would very much cut against the grain of Mueller and his experienced team of investigators and prosecutors. On the counterintelligence side, it is difficult to imagine that Trump’s account will be essential to assessing the threat that the Russians pose to our election process. And it is perhaps even just as likely that Trump’s testimony will cloud the matter with disinformation.

What’s more, Mueller would have to be concerned that immunizing Trump could lead to charges of overreaching, and might have unintended consequences. The question would be, why exactly was the special counsel taking this step? Would it be interpreted as signaling that Trump was in fact not a target of the investigation, which might then take a lot of air out of the probe? Or would it instead be used by those who know better to insinuate that Mueller was indeed on a “witch hunt,” intent on setting a perjury trap for Trump, hoping to catch him in a lie during his testimony? If there were subsequent impeachment proceedings, how would members of Congress view Trump’s immunized evidence? Would they dismiss it, because it had been compelled? And could a grant of immunity complicate state fraud prosecutions of Trump down the road? Because immunized testimony is compelled, state courts are likely to grant the same protections as are contained in the federal immunity statute, which might impede or even foreclose related state prosecutions in the future (Think: money laundering).

This is not to say that Mueller should worry primarily about the potential interpretation, misinterpretation, or criticism of his actions, because the reality is no matter what he does he will be misunderstood or criticized by some. But when considering an unorthodox investigative move, which is what immunizing Trump would be, he does have to consider whether the overall legitimacy of his work could be undermined.

At the end of the day, it is likely that in deciding whether to immunize Trump if he takes the Fifth, Mueller will start with this one simple question: will a grant of immunity advance either the criminal investigation or the counterintelligence investigation? If the answer is yes, then the question becomes: will the expected gains outweigh any potential costs or risks? Without knowing the details of what Mueller has uncovered to date, it is impossible to know the answers to these questions with any certainty, but it seems likely that immunizing Trump will lead to minimal or even no gains for Mueller’s investigations, and not likely any gains that could outweigh the likely costs of taking such a step.

One final thought: what about Trump? He has to calculate that he could take the Fifth and pay an enormous political cost for doing so—while Mueller may still force him to testify. Hence, he runs a double risk in refusing to testify, and he will not know what Mueller will do about immunity until and unless he takes the Fifth. As a political hand once said, if you are going to run for office again, you don’t take the Fifth. Trump’s choice set is, in significant respects, even more limited than most any witness or criminal defendant in this position.

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