GOP voters nominated the five-term congressman Tuesday. With 64 percent of precincts reporting, Kirk had 234,277 votes, or 58 percent.
Republicans have targeted the seat since then-Illinois Gov. Rod Blagojevich was arrested for trying to sell it more than a year ago. Blagojevich eventually appointed Roland Burris, who didn't run for a full term.
Illinois is being even more closely watched after an upset win by the GOP in Massachusetts that cost Democrats the late Sen. Edward Kennedy's seat.
The leading Democratic contenders are Illinois Treasurer Alexi Giannoulias, former Chicago inspector general David Hoffman and Chicago Urban League chief Cheryle Jackson.