SEIU Local 73 represents CPS support staff, serving in roles such as special education classroom assistants, bus aides, custodians as well as school security officers.
WATCH: Mayor Lori Lightfoot announces deal with SEIU Local 73 Sunday
Lightfoot joined CPS leaders at City Hall to give an update on bargaining talks Sunday night. She said the city and CTU weren't able to reach an agreement over the weekend despite the district offering what they called a "historic deal" for the union, including a 16% raise.
"We have put it in writing. Class sizes addressed, staffing is addressed, sanctuary schools are addressed and much much more," Lightfoot said.
CPS CEO Dr. Janice Jackson expressed disappointment Sunday over yet another day without a deal.
"We've given them a deal that guarantees social workers and nurses in every school everyday and pays the average teacher nearly $100,000 and reduces class sizes because we want to sign that deal right now and that is the deal that is on the table for CTU," Jackson said.
WATCH: CPS officials give update on contract negotiations with CTU Sunday
Shortly after, CTU leaders fired back in their own news conference.
"Please ask CPS if they can tell you if their deal offers class size protection for all students in this district," said CTU Vice President Stacy Davis Gates.
RELATED: Chicago Teachers Strike 2019: Where to find childcare when schools are closed
The union said they're only $38 million away from reaching a deal.
"It's one half of one percent of CPS' annual operating budget," CTU President Jesse Sharkey said. "We feel like we need to be able to get there. We feel like this is an amount of money that CPS needs to be able to make as an investment into our students and our classrooms."
But Chicago Public Schools put that number much higher at $100 million.
WATCH: The Chicago Youth Symphony Orchestra performs Sunday despite being out of class during Chicago teachers strike
At a Sunday church service in West Garfield Park, CTU members and supporters hosted a rally featuring several community and city leaders. Many took aim at Mayor Lightfoot.
"Somehow she has amnesia or something," said Vanessa Kotesky, a bilingual preschool teacher and CTU member. "She forgot what she said she was standing for, so we have to hold her feet to the fire."
Even though SEIU has reached a tentative deal, members said they'll be joining the picket lines Monday in support of CTU.
If the standoff continues past Tuesday, it'll mark the longest Chicago Teachers Union strike in over three decades.
Though it's not something either side said they want, University of Illinois Professor Robert Bruno said the union is intent on making a point.
"They made a statement in 2012. But in 2019, they're looking to achieve real gains embedded in the contract around staffing and class size, along with doing better financially," said Bruno, who co-authored a book on the 2012 strike. "They're really continuing that strike form 2012. I think they see it now as core, as central, that they need to get that out of the bargaining agreement."
Any agreement reached must first be voted on and approved by CTU members before classes can resume.
Last week, a judge ruled student athletes will not be able to compete in the state playoffs during the strike.
Judge Eve Reilly rejected their motion to allow kids to compete during the strike, writing, "It is not fair and it is wrong. However, the court must balance student safety above all other concerns."
Judge Reilly cited the potential conflict from coaches crossing the picket line, and potential dangers of having school facilities open but not adequately staffed.
More student athletes could be affected as the strike drags on, because a certain amount of games and practice are required to be eligible to compete.
RELATED: ACT testing postponed for CPS students Saturday as teachers strike continues
Multiple ACT exams that were scheduled to be administered Saturday also had to be postponed as the strike continues.
The exam is no longer required for Illinois high school students, but many students submit their scores when applying to college.