Another fired US Attorney was looking into Republican graft

Bud Cummins, the Arkansas US Attorney, speculates that he might have been fired because he was looking into "allegations that Missouri Gov. Matt Blunt had rewarded GOP supporters with lucrative contracts."

This was in 2006 when Missouri was holding the incredibly tight McCaskill/Talent Senate race.

By no means concrete, but it certainly adds to the "preponderance of the evidence".