Callis, who was Madison County Circuit Court's chief judge, has been encouraged by Democrats to run in the swing district. She left her position on Friday and is expected to announce a bid against Davis soon.

Davis narrowly beat physician David Gill (D) in 2012, and Democrats were nervous Gill, who'd lost before, would run again. But he was offered a job in state government by Illinois Gov. Pat Quinn (D) and accepted it on Friday, eliminating one potential primary foe for Callis in the district.

The district is a true toss-up: Davis won his race by fewer than 1,000 votes, about the same margin that Mitt Romney won the district by. But he's been off to a quick fundraising start this year and could be tough to beat in a midterm election.