On Wednesday, at a closed hearing of the House Judiciary Committee, former White House communications director Hope Hicks refused to answer questions about President Donald Trump’s attempts to sabotage the Russia investigation. Hicks brought a letter from White House Counsel Pat Cipollone, which declared her “absolutely immune from being compelled to testify before Congress” about anything that happened during her tenure. The letter is part of a White House campaign—reminiscent of President Richard Nixon’s “stonewall” strategy in the Watergate investigation—to defy congressional inquiries into whether Trump obstructed justice.

Trump’s claims that his former aides are “immune” from testimony are bogus. But they’re particularly bogus in the case of one key witness: Cipollone’s predecessor, former White House Counsel Don McGahn.

McGahn is the central witness in two crucial incidents: Trump’s attempt to fire special counsel Robert Mueller in June 2017 and the president’s effort in February 2018 to coerce a false statement about the attempted firing. On May 21, McGahn failed to show up at a hearing of the House Judiciary Committee, despite a subpoena to do so. Like Hicks, McGahn had a letter from Cipollone. “The President has directed Mr. McGahn not to appear” before the committee, the letter explained. But last week, Trump double-crossed McGahn by accusing him, in an ABC News interview, of having lied to Mueller about Trump’s obstructive acts. Trump also claimed to have witnesses who would contradict his former counsel.

Trump can’t have it both ways. He can’t attack McGahn’s testimony to Mueller and then refuse to let McGahn answer questions about it. The president can’t declare that he has witnesses and then refuse to produce them. Trump has forfeited any right to silence McGahn. Congress must call a hearing—with court-ordered enforcement of its subpoenas, if necessary—to resolve which man is telling the truth.

The president and his lawyers offer two reasons for blocking McGahn’s testimony. One is that McGahn has testified enough. “I’ve had him testifying already for 30 hours,” Trump told Fox News last month, referring to McGahn’s private interviews with Mueller’s investigators, which took place from November 2017 to February 2019. “It’s done,” said the president.

Trump’s second argument, detailed in a May 20 memo from Assistant Attorney General Steven Engel, is that allowing Congress to interrogate McGahn would violate bonds of trust and privacy between the president and his advisers. Such congressional “harassment,” the memo warned, might cause advisers to fear that any conversations with the president could be exposed and attacked. This, in turn, could lead the advisers to clam up and look out “for their own interests” rather than give the president “candid advice.”

Trump’s attack on McGahn in the ABC interview destroys these arguments. By creating a new dispute with his former counsel, the president has made clear that McGahn’s testimony to Mueller’s investigators was, in Trump’s view, insufficient to resolve the truth. The president has also accused McGahn of lacking candor and looking out for his own interests, not Trump’s. Any pretense of trust or privacy between these two men has been shattered by the president.

One thing is clear: Mueller’s report failed to resolve what happened between Trump and McGahn.

Mueller’s report, released in April, describes McGahn’s account of two phone calls from Trump to McGahn in June 2017, shortly after Trump learned that Mueller was directly investigating the president. According to the report, “McGahn was confident that he had at least two phone conversations with the President in which the President directed him to call the Acting Attorney General [Rod Rosenstein] to have the Special Counsel removed.” In the first call, “McGahn recalled that the President said something like, ‘You gotta do this. You gotta call Rod.’ ” In the second call, “McGahn recalled the President telling him ‘Mueller has to go’ and ‘Call me back when you do it.’ ”

In interviews with ABC’s George Stephanopoulos, recorded on June 11 and June 12, Trump ridiculed McGahn’s story. “I never suggested firing Mueller,” said Trump. Stephanopoulos reminded the president what he had said in the second phone call, according to McGahn: that Mueller “has to go.” Trump replied to Stephanopoulos: “I didn’t say that.” Trump dismissed McGahn’s whole story about the calls. “That never happened,” said Trump.

Trump told Stephanopoulos that McGahn could have been confused. But the president’s main bet was that McGahn had simply lied. Stephanopoulos asked, “Why would he [McGahn] lie under oath?” Trump replied, “Because he wanted to make himself look like a good lawyer.” Trump also claimed to have witnesses who could refute McGahn’s account. “I have people that will tell you it didn’t happen,” said the president.

One thing is clear: Mueller’s report failed to resolve what happened between Trump and McGahn. A central witness who refused to answer Mueller’s questions about obstruction during the investigation—the president himself—is now offering an account contrary to McGahn’s. This is a dispute between two White House officials over whether one of them committed acts that might constitute obstruction of justice. No claim of executive privilege or “absolute immunity” can entitle one of these men to silence the other.

In attacking McGahn’s story, Trump is also pitting himself against witnesses who interacted with Trump and McGahn immediately after the June 2017 phone calls. In those tense hours, McGahn told Reince Priebus, Steve Bannon, and Annie Donaldson—who were, respectively, the White House chief of staff, the White House chief strategist, and McGahn’s chief of staff—that he was resigning because Trump had instructed him to “do crazy shit.” Priebus and Bannon had a good idea of what had happened, because a few days before the Trump-McGahn phone calls, they had warned Chris Ruddy, a friend of the president’s, that Trump was strongly considering firing Mueller. Trump also phoned former New Jersey Gov. Chris Christie to discuss firing the special counsel. All of this is documented in Mueller’s report and attributed to interviews that Priebus, Bannon, Donaldson, Christie, and Ruddy gave to FBI agents under penalty of perjury. Every one of these witnesses can be called before Congress to help resolve the Trump-McGahn dispute.

In the ABC interview, Trump also challenged Bannon’s account of a separate incident. On May 16, 2017, a day before he was appointed as special counsel, Mueller went to the White House to speak with Trump about replacing James Comey, the FBI director Trump had just fired. Bannon later told investigators that “the White House had invited Mueller to speak to the President to offer a perspective on the institution of the FBI. Bannon said that, although the White House thought about beseeching Mueller to become Director again, he did not come in looking for the job,” according to Mueller’s report.

But in the ABC interview, Trump told a different story. “Robert Mueller wanted the job. He wanted to go back as the FBI director,” said the president. “But I told him no.” Trump dismissed Bannon’s contrary account: “Steve Bannon has no idea. Steve Bannon was not in the room.” Again, Trump claimed to have witnesses who could confirm his version of events. He assured Stephanopoulos, “I can get you two people that work in the White House office, in the Oval Office, to tell you [Mueller] was standing in line along with other people applying for the job.”

Great. Name the witnesses. Bring in Bannon, Mueller, and anyone else who claims to have been involved.

While we’re at it, let’s settle another question Trump has raised: the alleged fabrication of notes. On Tuesday, kicking off his reelection campaign at a rally in Orlando, Florida, the president bragged that his White House gave Mueller 1.4 million pages of documents to facilitate the Russia investigation. Some of the documents were contemporaneous notes taken by Donaldson and other White House aides. But in April, a day after Mueller’s report was released, Trump challenged the authenticity of those notes. “Statements are made about me by certain people in the Crazy Mueller Report … which are fabricated & totally untrue,” Trump tweeted. “Watch out for people that take so-called ‘notes,’ when the notes never existed until needed.”

Mueller’s report relies heavily on these notes. Trump’s accusation implies that the report, as a result, is factually wrong. So let’s bring in some of the former officials whose notes the president appears to be challenging: Donaldson, former White House chief of staff John Kelly, former White House staff secretary Rob Porter, and Jody Hunt, who was chief of staff to then–Attorney General Jeff Sessions. Kelly and Donaldson, in particular, took notes about conversations between Trump and McGahn on the question of firing Mueller. Are their notes forged?

Trump insists that Mueller didn’t settle these questions. So Congress must. Call the witnesses: McGahn, Kelly, Bannon, Priebus, Donaldson, Porter, Christie, and anyone else whose notes or testimony the president has disputed. If Trump has witnesses on his side, he’s welcome to name them. He’s also free to testify. What he’s not free to do is silence his accusers.