Republican Mark Kirk has captured the US Senate seat once held by President Barack Obama, dealing an embarrassing blow to Democrats.
Mr Kirk defeated Illinois Treasurer Alexi Giannoulias in one of the nation's ugliest and most closely watched Senate races.
Mr Giannoulias faced attacks over his family's failed bank, which gave loans to two men involved in organised crime.
Meanwhile, Mr Kirk was forced to apologise after the disclosure that he had made false claims about his military record.
The Illinois race got more attention than most Senate campaigns because this was the seat held by Mr Obama before his move to the White House.