From Politico: "The deal dismisses deadlocked charges against Manafort from an earlier bank- and tax-fraud trial in Virginia, but only after "successful cooperation” with Mueller’s probe into Russian election interference and whether the Trump campaign coordinated with Moscow on its efforts. Later, U.S. District Court Judge Amy Berman Jackson said Manafort is agreeing to "cooperate fully and truthfully" with the investigation.
However, a source close to the defense told POLITICO, "the cooperation agreement does not involve the Trump campaign. ... There was no collusion with Russia."
So Manafort appears to be willing to cooperate with Special Counsel regarding various players (some of whom are listed as unindicted co-conspirators) who were also involved in the various activities that led to charges against him. This would be very similar to the deal that Gates made for his plea. It wasn't that Gates was cooperating with Special Counsel regarding "Trump" as some initially assumed. But rather Gates was cooperating with special counsel regarding the connections between Ukraine and Manafort. It seems likely that more dominoes will fall within the Ukraine/U.S. lobbying world.
In other words...I might be more worried if my last name was Podesta right now, than if my last name was Trump.
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I think most everyone involved at this point (other than the true believers), has written off the idea that Mueller is going to prove some sort of Trump/Russia collusion. I think it's wildly speculative and wildly assumptive to believe that Manafort holds the key to proving such collusion.