His absured handling of the Illinois Senate race for one thing. Had a freshman poli-sci student been doing things around there, he couldn't possibly have done worse. From his insistence that Ryan step down, to his laughably bad handling of the Ditka situation, to the eventual joke that was Keys. It was a disaster, and he was either directly or indirectly part of all of it.
For another, the party in Illinois is obviously corrupt to the core, and you simply can't be as powerful as he is, and not know about it. He is either complacent or part of it.