The suburban Chicago district has long leaned Democratic; Sen. Mark Kirk (R-Ill.) had a number of tough races there during his time in the House. Dold narrowly won the seat in 2010 during the GOP wave, and was a top Democratic target in redistricting, when ticket-splitting voters were replaced with more reliably Democratic votes. President Obama won 63 percent of the district's votes four years ago.
Dold becomes the fourth House incumbent to fall in Illinois, partly due to a Democratic gerrymander. Illinois has been House Democrats' biggest bright spot on Election Night.