Kim Foxx, the Chicago prosecutor who faced intense criticism for her handling of the case of Jussie Smollett, the actor charged with staging a racist and homophobic attack, took a major step toward re-election on Tuesday with a victory in the Democratic primary.

Ms. Foxx’s closest rival, Bill Conway, a former prosecutor, conceded the race for state’s attorney in Cook County, Ill., which includes Chicago, in a call to the incumbent late Tuesday. With 82 percent of precincts reporting, Ms. Foxx had 48 percent of the vote, according to unofficial totals, and The Associated Press declared her the winner. Two other candidates were running well behind.

She will face a Republican opponent in November, Pat O’Brien, a former prosecutor and judge. But the county is heavily Democratic, and the party’s nominee typically wins countywide elections.

Ms. Foxx was elected Chicago’s chief prosecutor in 2016 with promises to change the criminal justice system, defeating her predecessor, who had come under heavy criticism for delaying prosecution of a white police officer in the death of a black teenager.