As President Trump’s impeachment trial begins, the question of who, if anyone, will testify remains unresolved — with much of the speculation swirling around John Bolton, the former national security adviser.

Democrats hope that Bolton, who left the White House on poor terms in September, will prove a devastating witness, should enough Republicans join Democrats to issue a subpoena for his testimony. He apparently referred to the plan to pressure Ukrainian President Volodymyr Zelensky, in search of dirt on former vice president Joe Biden, as a “drug deal” he wanted nothing to do with, and was reportedly one of three senior officials — along with Defense Secretary Mark T. Esper and Secretary of State Mike Pompeo — who met with President Trump to express their concerns about the withholding of military aid for Ukraine.

That may be why Trump said Friday that he’d assert executive privilege if Bolton were called to testify. “I think you have to for the sake of the office,” he told Fox News’s Laura Ingraham. Taking him at his word, Trump is suggesting that he will instruct Bolton not to answer certain lines of inquiry because doing so would set a precedent that could impede the ability of future presidents to keep communications with top aides confidential.

Bolton has given mixed signals about his willingness to testify. But if he wants to tell all, and the Senate wants to hear it, Trump almost certainly can’t stop him. Executive privilege is a shield that presidents have wielded to attempt to protect the executive branch from efforts by Congress, prosecutors or private litigants to force its employees (including former employees) to testify or provide documents. But we — a legal scholar and former executive branch lawyer who have studied and worked on these issues — are unaware of any instance in which an administration has asserted executive privilege to block the testimony of a former executive branch employee who wanted to talk.

What’s more, even if Bolton wanted to use executive privilege as an excuse to dodge some questions he’d prefer not to answer, he should not be able to, although the Senate will have the final say. As the Supreme Court has recognized, the validity of any assertion of executive privilege must be assessed by balancing the need for confidentiality against the need for the requested information. In the context of a Senate impeachment trial, the need to investigate credible allegations against the president far outweighs any confidentiality interests of the executive branch.

Divining Bolton’s intentions is admittedly a challenge. During the House of Representatives’ impeachment inquiry, he refused an invitation to appear and, through his lawyer, stated that he would challenge any subpoena. In a letter to a lawyer who represented both Bolton and his deputy, the Trump White House counsel argued that senior presidential advisers enjoy “absolute immunity” from being forced to testify. The House chose not to pursue the matter in court, most likely because litigation would last for months, if not years.

But last week, Bolton posted a surprising reversal on the website of his super PAC: “[B]ased on careful consideration and study,” he wrote, “I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify.

Guessing at what he’s up to is a Washington parlor game. Given that he plainly does not hold Trump in high regard, Bolton may be prepared to truly spill the beans. Or perhaps he wants to burnish his own reputation without significantly damaging the president — thus preserving his own perch in elite GOP circles. Bolton is nearly finished with a book that will reportedly cover his service to Trump, including the Ukraine imbroglio. He might be hoping to gin up publicity by revealing some details in his testimony, while saving the juiciest revelations for the book.

Let’s first assume that Bolton does want to reveal all he knows. If so, he would be in the same position as former FBI director James B. Comey, who eagerly testified to Congress at length about his private conversations with Trump. Setting aside congressional hearing rooms, it’s fairly routine for former presidential aides to talk publicly about conversations with their boss after leaving government. As an author, Bolton is following in the footsteps of several predecessors as national security adviser — including Zbigniew Brzezinski, Henry Kissinger and Susan E. Rice — who have written memoirs about their presidential service. Publications like these were surely subjected to prepublication review to ensure that classified information was not revealed. But unless the executive branch could show that exceptionally serious harm would arise from the revelation of such information, the First Amendment prevents a president from blocking publication.

The same broad constitutional freedom to speak applies if the former adviser were willingly revealing information in a Senate impeachment trial. (And Trump would need to surmount an additional hurdle even to get his day in court: Supreme Court case law grants the Senate nearly complete autonomy to decide how it will try impeachments.)

What about the scenario in which Bolton appears before the Senate but is not eager to reveal his most sensitive conversations? Trump could assert executive privilege as to, for example, any direct conversations that Bolton had with him (and any indirect communications via aides). Bolton might then happily defer to an objection by the president’s counsel and decline to answer those questions.

Chief Justice John G. Roberts Jr., who will preside over the trial, would then rule on whether Bolton must answer the question — or, alternatively, refer the issue to the Senate for a vote. If Roberts rules on the matter, any senator could ask that it be submitted to the full Senate, which could overturn Roberts by majority vote. We believe that the Senate should conclude that executive privilege provides no shield for Bolton to avoid answering specific questions. The purpose of executive privilege is to allow the government to keep its secrets when necessary, while allowing the other branches of government to do their jobs. It is intended to protect the lawful workings of the executive branch, not to shield misconduct from public view.

The Justice Department maintains that sitting presidents may not be criminally prosecuted, but that impeachment or electoral defeat are the only remedies for serious misconduct. It would fatally undermine the checks and balances in our constitutional system if the executive branch could deploy executive privilege to shield crucial evidence and effectively block impeachment of a president — especially one charged with seeking to corrupt the campaign for his reelection.

With little settled judicial precedent about executive privilege, most of the “law” in this area comes from the history of claims, counterclaims, disagreements, and accommodations between the executive branch and Congress. Notably, however, executive branch statements going back to the Washington administration recognize that executive privilege would not apply in the impeachment context. President James Polk stated that during impeachment inquiries, Congress could “penetrate into the most secret recesses of the Executive Departments … command the attendance of any and every agent of the Government, and compel them to produce all papers, public or private, official or unofficial.” (Predictably, Trump’s Justice Department has taken an aggressive position, suggesting that “the commencement of an impeachment inquiry only heightens the need to safeguard the separation of powers.”)

If the Senate asserted its appropriate constitutional powers to order Bolton to answer specific questions, Bolton might back down — or Trump might withdraw his privilege assertion in light of the (necessarily) bipartisan rebuke. If neither occurs, the Senate could escalate by voting to refer Bolton to the Justice Department for criminal contempt of Congress — though, for obvious reasons, it is unlikely that the agency would act on such a referral. Alternatively, the Senate might exercise its so-called “inherent contempt” power and decide to fine Bolton or even take him into custody until he agrees to testify or until the trial ends. Although Congress has not used this power since 1934, the Senate Watergate Committee’s threats to do so in 1973 helped persuade President Richard M. Nixon to allow aides to testify.

We don’t dare guess about whether Bolton will testify, what he will say if he does, or what Trump will do in response. But we can confidently assert that — unless Bolton is willing to risk going to jail for Trump — executive privilege should not stand in the way of his testimony.