Special Counsel Robert Mueller may have concluded that he cannot indict a sitting president, but that could all change if federal judges are given a chance to say what they think about the matter. A critical misstep by Trump’s lawyers could give the courts the opportunity to do just that. What’s the misstep? Litigating the question whether Mueller has the power to subpoena the president to testify before a grand jury.

As things currently stand, Mueller’s hands may be bureaucratically tied. “Justice Department memos going back to before Nixon say that you cannot indict a sitting president,” said Trump’s lawyer Rudy Giuliani last week, adding that the Special Counsel’s office “acknowledged to us orally that they understand that they can’t violate the Justice Department rules.” Regardless of whether Giuliani is to be believed about what Mueller’s team said, his description of the Justice Department’s Office of Legal Counsel (OLC) opinions—one written during the Nixon administration and the other during the Clinton era—is accurate. Simply put, those opinions conclude that a president is categorically immune from indictment while in office. What’s more, many legal experts, though notall, also think that Mueller is effectively bound by those opinions.

But the status of the opinions inside the Justice Department may not hold. It depends on how the fight over a subpoena plays out.

A problem for the President’s lawyers is that Mueller reportedly believes he has the power to subpoena the president to testify. There’s an untold threat to Trump’s presidency if his lawyers fight Mueller on that issue and lose in court. The end result could not only culminate in federal judges’ upholding the power of the special counsel to subpoena the president—it could also throw open the door to the idea that a sitting president is also subject to indictment and prosecution.

That’s because the two constitutional issues—a president’s immunity from indictment and a president’s immunity from a subpoena—are intertwined. Former Obama White House Counsel Bob Bauer wrote earlier this month that if Trump’s team fights the subpoena through litigation, they are likely to do so on the same grounds set forth in the Nixon and Clinton OLC opinions, namely, that subjecting the president to the criminal process would interfere with his ability to perform the duties of his office. One need look no further than Giuliani’s own arguments which contest Mueller’s ability to subpoena a sitting president based on the principles announced in the Nixon and Clinton memos. These connections are also to be found in the OLC’s own reasoning. A third OLC opinion, written during the Reagan administration, addressed whether a sitting President can be subpoenaed to testify in a criminal case. The author of that memo, Ambassador Douglas Kmiec, recently wrote about the logic of these opinions. “The evaluation of presidential subpoenas is also inseparable from the issue of whether a sitting president is subject to indictment,” Kmiec explained. He also wrote, “the OLC first concludes that a sitting president is immune from criminal prosecution and reasons from that that anything in support of that prosecution, including subpoenas for information, are not consistent with the separation of powers and the nature of the impeachment process.” If that’s correct, it may force a court to address the antecedent question—whether a sitting president is immune from criminal prosecution—in the course of addressing whether the president is subject to a subpoena in a criminal investigation. One caveat here is what Mueller would argue in the litigation. He would presumably content, at least in the alternative, that the president’s amenability to indictment–especially indictment during his term– is irrelevant to whether Trump can be subpoenaed to provide evidence about a grand jury investigation.

So then what might happen if Trump’s lawyers fight a subpoena in court? Mostlegal experts think that the odds are strongly in Mueller’s favor that the federal courts, on up to the Supreme Court, would validate the subpoena.

Such an outcome could shift the ground on which the President and the Special Counsel currently stand. Mueller’s hands might currently be tied by the two Justice Department memos stating that a sitting president cannot be indicted. But give federal judges the opportunity to cast doubt on the principles and reasoning that underpins those opinions, and what then? The judges would have ample opportunity to reach such questions when deciding on the constitutionality of the subpoena. Indeed, if Kmiec and Bauer are correct, the courts may not be able to avoid such implications.

The Nixon and Clinton memos are especially vulnerable because their reasoning is widely understood as weak and unsupportable. The opinions themselves also admitted to the lack of judicial decisions in direct support of their conclusions. That’s yet another reason that subsequent contrary statements from the bench is one thing that could break the memos’ bureaucratic hold on Mueller. Their status could become uncertain, even superseded by contrary judicial authority.

In the final analysis, Mueller has a strong hand in the constitutional battle over whether he could successfully subpoena the president to testify. If both sides understand the risks involved, Mueller would know he has the power to force the issue and Trump’s lawyers would stand down. If Trump’s team doesn’t, they may cost their client much more than just having to answer the special counsel’s questions.