After her defeat, Palmer ran for re-election to her State Senate seat and filed nominating petitions with 1,580 signatures on December 18, 1995—the last day to file nominating petitions.[8] Obama challenged Palmer's hastily gathered nominating petitions and those of the three other prospective candidatesNearly two-thirds of the signatures on Palmer's nominating petitions were found to be invalid, leaving her almost 200 signatures short of the required 757 signatures of registered voters residing in the Senate district; neither of the other three prospective candidates had the required number of valid signatures, leaving Obama, who had filed nominating petitions with over 3,000 signatures on the first filing day, as the only candidate to earn a place on the March 1996 Democratic primary ballot.[9][10]

Obama said that the challenges were necessitated by the obvious flaws in the challengers' signature sheets. "To my mind, we were just abiding by the rules that had been set up," he said in a 2007 interview with the Chicago Tribune.[10] When asked if voters were not disserviced by a ballot with no opposing candidates, Obama said "I think they ended up with a very good state senator


http://en.wikipedia.org/wiki/Alice_Palmer_(Illinois_politician)

In that case, I think he was just playing the game legally and it worked to his advantage. Basically, she decided to jump back in at the last minute. She waited too long and then didn't have time to go out and get enough signatures so she cheated (probably not her personally, it was probably her supporters who cheated). Not sure about the other candidates.