Democrat Sean Casten defeated Rep. Peter J. Roskam, the six-term incumbent Republican.

Casten’s win in a district considered solidly Republican could prove key for Democrats, who need to gain 23 seats to recapture the House.

The suburban Chicago district is one of the many suburban battlegrounds where Republicans are vulnerable because of a dislike of President Trump. Democrats hoping to recapture the House majority are looking to dozens of suburban districts nationwide to help them flip the chamber.

Democrats targeted Roskam early. His is one of many districts where Democrats hoped concerns about health care would help pull out a win.

Roskam voted for the Republican health-care bill to repeal and replace the Affordable Care Act. Casten ran several ads attacking Roskam over it.