The Associated Press projected Dold the winner late Tuesday night. It was a close race with Dold leading by only 3 points earlier in the evening.
The seat belonged to U.S. Rep. Mark Kirk who has won Barack Obama's Senate post.
Seals - who ran for the third time - hoped to become the first Democrat to hold the seat since Abner Mikva was elected 30 years ago. Early in the campaign, Seals was favored to win.
During the campaign, Dold likened himself to Kirk, a moderate Republican, hoping to appeal to voters in the district who tend to be socially liberal and fiscally conservative.
The race has been contentious with Seals trying to portray Dold as an ultra-conservative highlighting the fact that a right-to-life organization was supporting the Republican who is actually pro-choice.
The atmosphere at Dold's campaign headquarters in Winnetka was festive Tuesday night. Dold was at home watching returns with his family but was expected to arrive at the gathering.
The 10th Congressional District covers northern Cook and eastern Lake County.
