On Thursday, in an interview with Fox & Friends, President Donald Trump denounced Attorney General Jeff Sessions for recusing himself from the Russia investigation. “I put in an attorney general that never took control of the Justice Department,” Trump complained. He sketched a quid pro quo between getting the job and controlling the investigation, making clear that if he’d known Sessions was going to recuse himself, he wouldn’t have appointed him.

This kind of talk from Trump has become routine. With every unscripted comment, he makes clear that his aim is to quash special counsel Robert Mueller’s investigation. He doesn’t understand, or perhaps he doesn’t care, that this clear intent makes much of his behavior illegal. Nor does he understand how his own White House counsel, Don McGahn, has been helping Mueller make that case against him.

According to the New York Times, McGahn has spent some 30 hours with Mueller’s team, answering questions about what Trump has heard, said, and done behind closed doors. Trump’s lawyers say McGahn’s attorney has assured them the White House counsel didn’t implicate Trump in any crimes. But that assurance is worthless, because Trump’s team and Mueller’s team are using different definitions of obstruction of justice.

To make a case that Trump obstructed justice, Mueller can put together several episodes that demonstrate a pattern of “corrupt intent.”

Trump’s lawyers think obstruction entails a specific, indisputably illegal act. Trump is accused of doing several things that could meet that definition. One is allegedly asking then–FBI Director James Comey, in February 2017, to go easy on former National Security Adviser Michael Flynn. Another is firing Comey. Another is pressing Sessions to reclaim control of the Russia investigation. In a memo to Mueller earlier this year, Trump’s lawyers argued that none of these cases, in isolation, amounted to obstruction. They also maintained that nothing within the scope of Trump’s authority as president—firing the FBI director, for example—could possibly be illegal.

That’s the definition of obstruction that McGahn’s lawyer, Bill Burck, seems to have applied when he assured Trump’s attorneys that his client hasn’t implicated the president in a crime. The Times story says McGahn told Mueller’s investigators “that he never saw Mr. Trump go beyond his legal authorities.” A similar report, published Monday night by the Washington Post, says Burck assured Trump’s attorneys that McGahn “did not assert that Trump engaged in any wrongdoing.” The Post quotes an email from Burck, apparently to Trump’s attorneys, that says McGahn “did not incriminate” the president.

Trump’s lawyers seem satisfied with these statements. John Dowd, who served as Trump’s attorney during an earlier stage of the investigation, says that he debriefed McGahn and Burke and that McGahn was a “terrific witness” for Trump. Rudy Giuliani, Trump’s new lawyer, says McGahn testified that “everything was legal” and that “the president didn’t do anything wrong.”

But that doesn’t matter. Mueller’s people don’t care about whether McGahn thinks Trump’s behavior was illegal. They can already infer, from McGahn’s choice to remain in his job, that he thinks everything Trump did can be defended. They’ve interviewed McGahn because he’s a direct witness to key events in the White House—moments in which Trump may have said or done things that revealed his motives. If McGahn highlighted a single, Nixonian remark in which Trump decisively incriminated himself, that’s a home run for the special counsel. But it isn’t necessary. To make a case that Trump obstructed justice, Mueller can put together several episodes that demonstrate a pattern of “corrupt intent.”

Corrupt intent is a fuzzy standard. In this context, it means that even when you’ve done something that isn’t inherently illegal—firing the FBI director, for example—it becomes obstruction of justice, and therefore illegal, if you did it for a corrupt purpose. Criminal codes and court rulings don’t do much to clarify the term—they refer to “an improper purpose” or “corruptly” seeking “to influence, obstruct, or impede, the due administration of justice”—but former prosecutors say that in practice, corrupt intent is demonstrated by collecting “statements by the defendant that directly reveal” his motives. Last year in Slate, Samuel Buell, the former lead prosecutor for the Justice Department’s Enron Task Force, explained that this kind of evidence has been used successfully to prove that people were guilty of corrupt intent when they sought “to derail or even slow a criminal investigation in order to protect associates.”

Prosecutors and legal experts point to several episodes in the Trump obstruction case that could show corrupt intent: the meeting with Comey about Flynn, the firing of Comey, the construction of rationales for firing Comey, Trump’s pressure on Sessions to retake control of the investigation, Trump’s attempts to block legislation that would have protected Mueller, Trump’s attempts to fire Mueller, and Trump’s use of pardons to signal that he would protect witnesses who protected him.

McGahn was directly involved in nearly every one of these episodes. He’s the guy who told Trump in January 2017 that Flynn had misled the FBI about back-channel talks with Russia—which means that Trump, when he later asked Comey not to pursue Flynn, knew he was tampering with an investigation. He’s the guy who lobbied Sessions, on Trump’s orders, not to recuse himself. He’s the one who gave Trump the bad news that Mueller was being appointed—and who then watched as Trump, in a White House meeting, exploded at Sessions for failing to protect him. He’s the guy Trump repeatedly told to fire Mueller. McGahn also edited the letter in which Trump explained why he was firing Comey. And he was Trump’s conduit for issuing pardons.

Mueller is keenly interested in these episodes. We know this because Trump’s lawyers reported as much in their memo earlier this year. “In our conversation of January 8, your office identified the following topics as areas you desired to address with the President,” the memo begins. Then it lists 16 subjects. Ten of them are obstruction-related, including Trump’s meeting with Comey, Trump’s reactions to learning of the FBI’s Russia investigation, Trump’s reaction to Sessions’ recusal, Trump’s conversations with other intelligence officials, Trump’s reasons for firing Comey, Trump’s comments after firing Comey, and Trump’s reactions to Mueller’s appointment.

That’s what Mueller’s people asked McGahn about in their interviews. According to the Times, Trump’s attempts to fire Mueller “would not have been revealed to investigators without Mr.

McGahn’s help.” In addition, McGahn

provided the investigators examining whether Mr. Trump obstructed justice a clear view of the president’s most intimate moments with his lawyer. Among them were Mr. Trump’s comments and actions during the firing of the FBI director, James B. Comey, and Mr. Trump’s obsession with putting a loyalist in charge of the inquiry, including his repeated urging of Attorney General Jeff Sessions to claim oversight of it. … Mr. McGahn gave to Mr. Mueller’s investigators, [sources] said, a sense of the president’s mind-set in the days leading to the firing of Mr. Comey; how the White House handled the firing of the former national security adviser, Michael T. Flynn; and how Mr. Trump repeatedly berated Mr. Sessions, tried to get him to assert control over the investigation and threatened to fire him.

The Times says Burck tried to explain to White House lawyers that such disclosures could hurt Trump “even if the president did nothing wrong,” because Trump “probably made politically damaging statements to Mr. McGahn as he weighed whether to intervene in the Russia investigation.”

Trump and Giuliani are obtuse to this distinction. In his rant to Fox & Friends this week, Trump displayed his inability to grasp, or at least his indifference to, the idea that ulterior motives can make an otherwise legal act corrupt. Giuliani, too, has insisted in multiple interviews that motives don’t matter. And on Monday, Giuliani brushed off the Times story, saying McGahn “didn’t offer anything illegal.”

If Mueller’s report ends up presenting a pattern of corrupt intent, Trump and Giuliani will be just as incredulous as they are now. They’ll say it doesn’t add up to obstruction, and Senate Republicans will probably agree with them. But by then, a Democratic House may have voted to impeach the president. And if that happens, you can thank Don McGahn.

Read more from Slate:

• We Don’t Need an Investigation to Know Trump Betrayed His Country. All the Evidence Is Right Here.

• A Holdout Juror Made It More Likely Manafort Will Go to Jail Even if Trump Pardons Him

• A Georgia County’s Flagrant Attempt to Disenfranchise Black Voters Collapses in the Face of Public Outcry

• Fox News’ Fixation on Mollie Tibbetts’ Murder Is Astonishingly Hypocritical