The guilty plea of former National Security Adviser Michael Flynn — the fourth official from President Trump’s campaign implicated in criminal activity — is in several ways even more damaging to the president than special counsel Robert Mueller’s charges earlier this fall against Paul Manafort, who actually ran Trump’s campaign for a time. In short: Flynn’s plea is a sign that Mueller’s investigation has reached a more central place within Trump’s orbit, and that it could creep closer to Trump still. But let’s go through why in more detail.

Flipping — Mueller is charging Flynn only with perjury, even though there have been a number of other allegations against him, particularly about his lobbying work. Flynn’s guilty plea suggests that he is cooperating with Mueller’s investigation. In fact, ABC News is reporting that Flynn has told prosecutors that he is willing to provide potentially damaging information about the president. Indeed, Flynn was a top adviser on both the campaign and, briefly, in the White House. He could have damaging information about very senior officials in the administration, potentially including senior adviser Jared Kushner, the president’s son-in-law, and Trump himself.

Recency — According to the court documents, Flynn lied to the FBI about his contacts with Russia’s U.S. ambassador at the time, Sergey Kislyak, while working as Trump’s national security adviser. Trump aides tried to brush off the charges against Manafort and his aide Rick Gates by emphasizing that much of the activity that Mueller highlighted came before either man was on Trump’s campaign. (Some of the charges dated to activities by Gates and Manafort as far back as 2006.) But the documents describe events that happened in December and January — when Flynn was working on Trump’s transition team and in the White House.

The White House –– Unlike Gates, Manafort or George Papadopoulos (the Trump foreign policy campaign adviser who has pled guilty to lying to the FBI about his interactions with Russian figures during the election), Flynn followed Trump into the White House and took one of the most important posts in a presidential administration. His tenure was very short, as he was pushed out amid the controversy over his contacts with Kislyak. (After November’s election but before Trump took office, the Obama administration imposed new sanctions on Russia in retaliation for Moscow’s alleged election interference. According to the court documents, Flynn, in a conversation with Kislyak, urged Russia not to retaliate for the Obama administration move. The implication was that the Trump administration would reconsider and potentially reverse the sanctions.)

Trump and his aides have downplayed the president’s connections with Manafort and Papadopoulos. That is not possible with Flynn.

Obstruction of justice –– Remember that former FBI Director James Comey, in the statement he released the day before he testified on Capitol Hill in the spring about the circumstances of his dismissal, described a one-on-one Oval Office conversation with Trump. The president, according to Comey, said, “I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy.” Comey interpreted this statement as asking him to end the FBI’s investigation of Flynn, which Comey refused to do.

If you believe Comey’s account, the man whom the president was concerned enough about to personally ask for him not to be investigated has now plead guilty. And if Mueller is investigating Trump for obstruction of justice, as has been reported, that case is reinforced by this plea. Trump fired Comey after the FBI director would not stop investigating someone (Flynn) who was in enough legal jeopardy to later plead guilty.

Also, the obstruction case against Trump got stronger in the hours just before the Flynn announcement. The New York Times reported on Thursday night that the president has called several GOP senators and asked them to do what they can to wind down the various Russia investigations on Capitol Hill. One of the senators that Trump reached out to is North Carolina’s Richard Burr, who is leading the Senate Intelligence Committee probe. It is not necessarily illegal for Trump to call Burr and other senators and make that request, but his behavior fits with a broader pattern of moves, like Trump’s alleged conversations with Comey, that suggest Trump is trying to stall or slow down the various investigations into his campaign’s connections with Russian officials.

Of course, we have only so much visibility into Mueller’s investigation. We don’t know everything he does, and so can’t be sure how this will play out. But based on what we do know so far, Flynn’s plea is an important milestone in the investigation, it’s bad for Trump, and it may portend worse.