CBS (CHICAGO)–Cook County Board President Toni Preckwinkle announced Thursday her decision to enter the already-crowded race for Chicago mayor, but her announcement was clouded by the controversial firing of one of her top aides this week.

Preckwinkle, 71, who serves as head of the Cook County Democratic Party, held a rally today at the Chicago Lake Shore Hotel in East Hyde Park.

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Cook County Board President Toni Preckwinkle announced Thursday her decision to enter the Chicago mayor’s race.

The field of mayoral candidates has grown denser in the weeks since Mayor Rahm Emanuel announced he was dropping his bid for a third term to spend time with his family.

Her campaign kickoff on Thursday comes one day after announcing she had forced out her chief of staff, John Keller, for engaging in “inappropriate behavior on his personal time.”

Preckwinkle said she learned about the allegation against Keller on Friday, and when she confronted him, he did not deny it. She then demanded and received his resignation on Tuesday. Keller is said to have worked for a political campaign for someone else when the alleged harassment occurred.

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“I have zero tolerance toward harassment of any kind,” she said Thursday.

Toni Preckwinkle (CBS)

Preckwinkle is the second high-profile candidate to jump into the race for mayor this week. Gery Chico, who ran against Emanuel in the 2011 race for mayor, threw his hat in the ring on Tuesday.

Chico, who served as longtime Mayor Richard M. Daley’s chief of staff, president of the Chicago Board of Education, and head of the Chicago Park District and City Colleges of Chicago, has said those experiences make him the most qualified candidate in the race.

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At least a dozen other candidates have announced bids for mayor, including former White House chief of staff and Commerce Secretary Bill Daley, Cook County Circuit Court Clerk Dorothy Brown, former Chicago Public Schools principal Troy LaRaviere, former Chicago Police Board president Lori Lightfoot, former Chicago Police Supt. Garry McCarthy, former CPS Chief Executive Officer Paul Vallas, millionaire businessman Willie Wilson, policy consultant and former 2015 mayoral candidate Amara Enyia, community activist Ja’Mal Green, attorney Jeremiah Joyce Jr., former City Council candidate John Kozlar, DePaul University student Matthew Roney, and tech entrepreneur Neal Sales-Griffin.