Lightfoot's office said she had a scheduling conflict that prevented her from attending.
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The 11 other candidates were asked about their solution's to the city's problems.
"We can't have the next mayor think outside the box. I say rip up the box," said candidate Frederick Collins.
"As mayor, I would definitely lead with listening and act with compassion," said candidate Sophia King.
RELATED | Ald. Sophia King says she would prioritize safety, collaboration if elected Chicago mayor
The forum was held at a North Side synagogue. Candidates offered ideas on how to fight homelessness, improve education, and reduce hate, antisemitism and crime.
"You've got to return to community-based policing so that, you know, the cops, the beat cars are there, and the police officers know the communities they serve," said candidate Paul Vallas.
"The beginning or the ending answer isn't police. It's about repairing broken neighborhoods," said candidate Ja'Mal Green.
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The current field of at least 16 announced candidates will get even more crowded Thursday morning when Congressman Jesus "Chuy" Garcia launches a second mayoral bid. The excepted announcements comes 40 years to the day Harold Washington announced his run for mayor.
READ MORE: Congressman Jesus 'Chuy' Garcia running for mayor of Chicago, sources say
Before appearing at the forum, Cook County Commissioner Brandon Johnson was endorsed by SEIU, one of the state's largest unions.
"Once I'm mayor of Chicago, every single voice that has been ignored, that has been denied, your voices will be heard," he said.
SEE ALSO | Cook County Commissioner Brandon Johnson enters Chicago mayor election
SEIU joined the Chicago Teachers Union in endorsing Johnson. Both unions endorsed Chuy Garcia in his 2015 run against Rahm Emanuel.