CHICAGO (CBS) — With a little more than three weeks left until the runoff election, both candidates for Chicago mayor faced off in a forum Thursday night, and it didn’t take long for things to get heated.

(Credit: WMAQ)

They fiery exchange between mayoral candidates Lori Lightfoot and Toni Preckwinkle could be a sign of what’s to come leading up to the April 2 runoff.

The forum came just hours after Lightfoot received the backing of the Chicago Firefighters Union, which could reflect wider Northwest Side and Southwest Side support.

Aldermen Nicholas Sposato and Anthony Napolitano, both former firefighters who Preckwinkle quickly criticized, were at that announcement.

“Both of whom are pro-Trump Republicans and anti-immigrant alderman,” Preckwinkle said. “That’s profoundly disturbing to me since she claims to be a progressive.”

Preckwinkle voiced those same criticisms during the forum leading to a heated exchange.

“No, no, no, you’re spewing,” Lightfoot said.

“No, let me finish,” Preckwinkle said.

“Fine,” said Lightfoot. “I’m going to let you lie and then I will respond.

“You know I’m not accusing you of lying,” Preckwinkle said.

“No, you are lying,” said Lightfoot. “There’s no lie that you clearly will not tell. It is sad and pathetic.”

“Just really disrespectfull,” said Preckwinkle.

While both candidates held their own in the forum, a new Lightfoot poll of 799 voters shows her with a substantial lead of Preckwinkle at 59 percent to 29 percent.

And hoping to expand that lead, Lightfoot released a new ad highlighting Preckwinkle’s ties to the Democratic party, including hiring Ald. Ed Burke’s son for a six figure county job.

Friday could bring a key endorsement for either Preckwinkle or Lightfoot. Businessman Willie Wilson, who won more wards than any mayoral candidate in the election, is expected to announce who he’s supporting for mayor.