Donald Trump has a habit of incriminating himself. No one understands this better than the people who work for him. In Bob Woodward’s new book, Fear, Trump’s aides reportedly describe how they’ve scrambled again and again to stop him from exposing the extent of his paranoia and dishonesty. But the president has thwarted them. He can’t stop talking about himself, especially on Twitter. And when he does, he broadcasts his corruption. The author who’s going to bring down Trump isn’t Woodward. It’s Trump.

Woodward’s book, as outlined in the Washington Post, adds to the pile of evidence that Trump has lied about and sought to obstruct the Russia investigation. The book reports that in May 2017, Trump erupted at the appointment of special counsel Robert Mueller, saying, “Everybody’s trying to get me.” It says Trump called Attorney General Jeff Sessions a “traitor” for recusing himself from the investigation. This backs up previous reporting that Trump sought, by hounding and pressuring Sessions, to gain partisan control of the investigation.

The book also reveals that Trump’s lawyers tried to stop him from granting an interview to Mueller because they recognized that Trump couldn’t keep his story straight. When the lawyers grilled Trump in a rehearsal, he responded with “stumbles, contradictions and lies,” according to the Post’s summary. CNN, quoting another line from the book, says the lawyers later told Mueller that Trump couldn’t go through with the interview because during their mock interrogation, “He just made something up. That’s his nature.”

Trump’s attorneys worry that Mueller might recommend perjury charges against the president. But the more serious charge would be obstruction of justice. Trump is already implicated in a series of incidents that seem designed to impede the investigation: asking then–FBI Director James Comey to go easy on former national security adviser Michael Flynn, pressing Comey to “lift the cloud” of the Russia investigation, firing Comey, pressing Sessions to reclaim control of the investigation, and attempting twice to fire Mueller.

Together, these incidents could add up to obstruction. But to make that case, Mueller has to show that Trump acted with “corrupt intent.” As Trump’s lawyers explained in a January memo to Mueller, the president is constitutionally entitled to fire the FBI director, the attorney general, and anyone in the Justice Department who refuses to get rid of a special counsel. Mueller has to show that Trump did or threatened these things not for the good of the country, but to shield himself or his friends from justice. According to Woodward’s sources, Mueller has told Trump’s lawyers that he needs the president’s testimony because “I want to see if there was corrupt intent.”

The incident Woodward reports, in which Trump allegedly called Sessions a “traitor” for his recusal, helps Mueller make that case. When combined with another episode cited in the book—Trump accusing then–economic adviser Gary Cohn of “treason” for attempting to resign—it suggests that Trump defined treason as betrayal of himself, not betrayal of the United States. But it doesn’t establish Trump’s motivation beyond doubt. To prove corrupt intent definitively, you’d have to catch the president attacking the justice process specifically because it threatened him or his political allies.

There’s no handy way to spin this tweet.

On Monday, Mueller received that evidence. He didn’t get it from Woodward or anyone quoted in the book. He got it from Trump. And it wasn’t in an email or a secret audio recording. It was on Twitter. There, the president lambasted Sessions for allowing the Justice Department to indict two pro-Trump congressmen: Chris Collins, charged with insider trading, and Duncan Hunter, charged with stealing campaign money for personal use. “Two long running, Obama era, investigations of two very popular Republican Congressmen were brought to a well publicized charge, just ahead of the Mid-Terms, by the Jeff Sessions Justice Department,” Trump wrote. “Two easy wins now in doubt because there is not enough time. Good job Jeff…”

There’s no handy way to spin this tweet. You can’t argue, as Trump’s attorneys have in the Comey case, that Trump fired an official for malfeasance and knew his replacement would continue the investigation. In the tweet, Trump explicitly denounced the Justice Department for bringing charges based on thorough investigations. He made clear his objection: that the charges hurt his party. And he complained that the indicted congressmen were “very popular.” That’s crucial. It shows that Trump wasn’t saying he wished the congressmen had been indicted sooner so that other Republicans could have replaced them on the ballot. Trump was implying that he wished the congressmen had been allowed to carry the Republican banner through the election without being indicted at all.

This strengthens the case against Trump by broadening it. It shows a pattern of corrupt intent that goes beyond the Russia investigation. Trump’s lawyers have long argued that Trump knows he didn’t collude with Russia and therefore genuinely believes, correctly or not, that the investigation is a sham. But Trump, by all appearances, knows nothing about the Collins and Hunter cases. All he knows is that both men are Republicans, that they were the first two congressmen to endorse his candidacy, and that they’re under indictment. He has no plausibly innocent grounds on which to attack the indictments.

As the gravity of Trump’s outburst becomes clear, you’ll hear his attorneys and surrogates brush it off. They’ll say that tweets prove nothing about obstruction of justice, because tweets are just words—Sessions still has his job—and tweets are public, which is the opposite of a conspiracy. But none of that matters. Legally, the tweet sheds light on a specific question: Trump’s attitude toward the administration of justice. It shows that his motivation in attacking investigations is corrupt. It illuminates everything—the scramble to protect Flynn, the efforts to corrupt Comey, the hounding of Sessions, the attempts to fire Mueller—that happened behind closed doors.