It's commonly cited that Obama could easily lose the 2004 Senate Election in Illinois if Jack Ryan's divorce controversy doesn't stir things up.
I don't think that's the case myself - Illinois's been a pretty blue state in this age. The seat had been mostly blue in the past, held by Alan J. Dixon and then Carol Mousely-Braun, who might've kept the seat blue if she hadn't run in 1998 against Peter Fitzgerald, despite a strong number of scandals on her part. In addition, despite her scandals, she still lost within 3% to Fitzgerald in 1998.
I don't imagine Obama might've won by the margin he did, but I think he still would've stood a chance.
(This is kind of a general catch-all WI thread for this particular state senate election.)