Rezko has had close ties to U.S. Sen. Barack Obama and has become an issue in the Democratic presidential battle. Obama has returned Rezko's campaign contributions.

The Chicago Tribune identified "Co-schemer A" as William Cellini, a Republican power broker in Illinois. Prosecutors say that he worked with Stuart Levine, expected to be a prosecution witness in Rezko's trial, to get kickbacks from firms that wanted to do business with the state government.

The government made the claim in a filing to counter a defense motion to keep some evidence out of the trial. Rezko's lawyers argued that the prosecution case depended heavily on an unreliable witness, Levine.

Rezko was also a major fundraiser for Illinois Gov. Rod Blagojevich.