Paul Manafort outside the federal courthouse in Washington in February. (Shawn Thew/EPA-EFE/Shutterstock)

Randall D. Eliason teaches white-collar criminal law at George Washington University Law School.

Former Trump campaign chairman Paul Manafort pleaded guilty to two counts of conspiracy on Friday in a D.C. federal court. The real bombshell was not the guilty plea itself, but the fact that the deal includes an agreement that Manafort will cooperate with federal prosecutors. This is a huge leap forward for the investigation of special counsel Robert S. Mueller III — and an ominous sign for President Trump.

From Manafort’s perspective, a plea arrangement makes perfect sense. Manafort was convicted of eight felonies in Alexandria last month, and faces close to a decade in prison on those counts. He was charged with seven more felonies in the District case and the evidence against him appeared overwhelming. By pleading guilty to just two counts, he avoids the expense of a trial. More significantly, sentencing guidelines in the D.C. case would have called for about 20 years in prison if he had been convicted, but his guilty plea on Friday caps his possible sentence at 10 years.

But the personal impact felt by Manafort was overshadowed by the implications of his agreement to cooperate. The big mystery with Manafort has always been the reasons behind his refusal to cut a deal. When he held fast and stood trial in Alexandria, it began to look as though he would never turn against the president. Many speculated that Manafort might be holding out for a presidential pardon. Trump praised Manafort as a “brave man” while condemning those who cooperate with prosecutors as “rats.” On the eve of his second trial, though, Manafort finally seems to have accepted the reality of his situation: As a 69-year-old man convicted of multiple felonies, fulsome cooperation with prosecutors could mean the difference between spending the rest of his life in prison and someday walking free.

As the White House was quick to point out, the charges to which Manafort pleaded guilty do not directly relate to Trump or his campaign. But cooperation agreements are not limited to cooperation in the same case. Prosecutors would have no reason to seek Manafort’s cooperation in connection with the charges listed in his D.C. indictment; Manafort was the primary defendant and his plea largely resolves that case. The existence of the agreement — and the favorable terms offered by prosecutors — suggest Manafort can provide useful information about other aspects of Mueller’s investigation.

What’s more, prosecutors don’t enter into such agreements blindly. The agreements typically are preceded by extensive debriefings that allow prosecutors to see what the witness has to offer and to assess the credibility of the information. Mueller’s team apparently found Manafort’s information important and credible enough to be worthy of a deal — and a pretty sweet deal at that.

Contrary to some initial reports, there is no carve-out in the cooperation agreement for matters involving the campaign. Those who cooperate with federal prosecutors don’t get to pick and choose the subject matter. Either you agree to tell prosecutors everything you know about whatever topics the prosecutors are interested in, or there’s no deal. Manafort’s agreement requires him to cooperate “in any and all matters as to which the Government deems the cooperation relevant.” And if Manafort lies or tries to protect the president (or anyone else), the deal is off, and prosecutors are free to bring back all charges against him.

Trump’s lawyer Rudolph W. Giuliani has professed nonchalance about whether Manafort cooperates, claiming the president’s team isn’t concerned because the president did nothing wrong, and that Manafort is an “honorable man.” But despite that bravado, this is grave news for the president.

Manafort was at the center of Trump’s campaign during key events in Mueller’s investigation of whether the campaign conspired with Russians to influence the election. Manafort came to the campaign with extensive ties to Russians and Russian-connected individuals. He attended the June 2016 meeting at Trump Tower with Russian representatives offering “dirt” on Democratic candidate Hillary Clinton. He was campaign chairman during the Republican National Convention when the party platform was changed to adopt a more pro-Russia stance on Ukraine — one source of suspicions about a possible quid pro quo between the campaign and Russia. In short, Manafort is in a position to know a great deal about matters at the heart of Mueller’s investigation.

Mueller has pursued a classic, painstaking white-collar investigation, building cases against lower-level players and persuading them to “flip” against bigger fish. The pursuit of Manafort took more than a year, but Mueller has now landed his biggest fish so far. Prosecutors typically offer cooperation deals to those who can deliver the goods on someone higher up the food chain — and the list of those above Manafort is not very long.