Probably he has agreed to cooperate with the special counsel because he’s holding out for a reduced sentence.

In federal sentencing practice, the prosecutor often holds the keys to a defendant’s prison. If the defendant cooperates with the prosecutor to a degree that the prosecutor finds valuable—what the law calls “substantial assistance”—then the prosecutor can file a motion in court to reduce a defendant’s term of imprisonment. Inadequate or incomplete cooperation? Then there is no motion. It’s a powerful incentive.

Notably, President Trump has in the past condemned cooperating defendants as “flippers.” He even went so far as to congratulate Manafort for not “break[ing].”

Now, it seems, Manafort has broken. Under the terms of his plea agreement Manafort is obliged to cooperate “fully, truthfully and completely.” More to the point, no prosecutor of any experience would enter into a cooperation agreement if he did not already know what the witness was likely to say. Prior to the plea, Manafort certainly sat down with the special counsel’s office to reveal what he knows.

And so, the special counsel now has the cooperative testimony of Trump’s former campaign chairman—and that is likely to make Trump uncomfortable. Two issues will probably be at the forefront of the special counsel’s inquiry: First, and most obviously, Manafort was also at the infamous Trump Tower meeting with Russians who were offering to discuss “dirt” on Hillary Clinton. His testimony about that meeting will offer an independent confirmation of what took place and he may also know about internal campaign communications relating to the meeting, both in preparation for it and after it had occurred.

Second, and more speculatively, Manafort’s connection to Ukraine’s pro-Russian party makes it likely (though not certain) that he played a role in the changes made to the Republican national platform. As originally proposed in early 2016, the Republican party platform called for providing Ukraine with "lethal defensive weapons"—something that almost every Republican in Congress supported at the time. At the urging of the Trump campaign, the phrase was changed to "appropriate assistance”—a far more anodyne promise. No doubt Mueller will want to explore this change with Manafort and determine, as well, what role (if any) Trump played in requesting it.

All of which brings us to the $64,000 question—and what about a pardon? Before his guilty plea many saw Manafort as making a pitch for a presidential pardon. In staying strong and refusing to break, he was, in effect, asking for the president’s help. Now, with his cooperation agreement signed, a pardon seems less likely.

Manafort has taken off his MAGA hat and put on a Team Mueller cap.

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