Anthony Williams isn’t a Democrat. In 2008, he was the republican nominee for the U.S. House, in Illinois’ 2nd Dist. His website states that he’s pro-tax cuts, pro-life, and pro-gun rights.
Williams states on his website that he switched parties to oppose Jack$$on. Therefore, he ran as a Democrat in 2012.
Switching parties for electoral reasons is not unheard of. Just look at Michael Bloomberg and Arlen Specter.