Bob Dold, (R) has reclaimed the seat he narrowly lost to Brad Schneider (D) in 2012.
The former congressman, a Republican businessman from Kenilworth, won the 10th Congressional District race 52 percent to 48 percent, according to unofficial numbers.
One of the few contested congressional races in the country, the battle for the North Shore district was closely watched throughout the campaign, with outside money playing a key role.
Each candidate served one term in Congress. Dold replaced Sen. Mark Kirk and narrowly lost the seat to Schneider two years ago.
Kirk repeatedly stumped for Dold this election, releasing a campaign video that declared, "He's like me," and appearing at rallies.
Dold also got some help from former New York City Mayor Michael Bloomberg's political action committee, Independence USA. The Super PAC poured $1.9 million into a television ad buy backing the candidate.
Schneider saw some big-name backing too, but it wasn't enough.
"I didn't run to be a Congressman," Schneider said. "I wanted to be in Congress to tackle our problems. ... tonight is not the end."
Vice President Joe Biden voiced his support for Schneider during a second visit to Illinois within five days last month. He claimed the GOP "don't understand how hard people are working."
Both Dold and Schneider spent millions on attack ads in this rematch of the 2012 race. They disagreed on the Affordable Care Act, Medicare cuts and a minimum wage hike.
Much like the governor's race, this one was deemed too close to call for much of the election season, and turnout was expected to be the deciding factor.