Lewis said that after School Board President David Vitale entered talks Thursday, union leaders were told they would get a proposal "addressing some of our biggest issues and we did not."
Vitale, who a day earlier expressed optimism, warned parents to prepare for a strike as he left negotiations at union headquarters.
"Parents need to plan for Monday morning, and we will start to execute our (strike contingency) plan because logistically it takes us time to do that," he said. "It's not a statement that we're not going to get there it's just that we're being cautious and precautionary about Monday."
Lewis said weekend talks, which begin at noon Saturday, would be "intense."
The sticking points to a new contract remain raises, a recall policy for laid-off teachers, and a new teacher evaluation system.
It wasn't clear exactly how far the two sides were apart on salary hikes. CPS' last known offer was for 2 percent increases in each year of a four-year contract. The union had been asking for significantly more, although Lewis declined to say Friday how much it had come down from its call for a 19 percent raise in the contract's first year.
The district has backed off its call for merit pay in the face of staunch union opposition but is still insisting that annual raises not be given for experience.
The union is also concerned about the district's plan to close schools - Lewis has said that 100 schools could be shut down in coming years - and are pushing hard on a recall policy for laid-off teachers.
"At the end of the day, both sides needs to craft a contract that they can claim is a win," said Robin Steans, executive director of education policy group Advance Illinois. "What the rest of us need to pay attention to is whether it's a contract that's good for kids. Hiring great people is the single most important thing principals do for their students."
If a deal is reached, the union said it needs to gather its delegates to call off a strike. A union official said she expects a "reasonable cut-off of negotiations on Sunday," and if a deal is reached, a delegate vote can be held within an hour.
Vitale said Thursday he thinks a deal can be made without Mayor Rahm Emanuel coming to the table. The mayor's point person on education, Beth Swanson has been at the negotiating table for the last week, Gadlin said.
Some observers saw Vitale's appearance at the negotiating table Thursday as a reason for optimism. Former interim schools chief Terry Mazany went so far as to call it "a game changer."
"David Vitale is deeply knowledgeable of the Chicago Public Schools and he has professional experience leading organizations and unions through massive change," said Mazany who heads the Chicago Community Trust.
Union officials Thursday agreed that Vitale's entry into the talks was "encouraging."
With talks going down to the wire, advocacy groups for both sides are pushing to win public support.
Democrats for Education Reform, an organization started by Wall Street hedge fund managers that opened offices in Chicago earlier this year, staged a protest near union headquarters Friday by a group of 50 parents, students and pastors who have supported many CPS initiatives like school closings.
"Call off the strike," said Rebeca Huffman, executive director of the group, which also goes by the name Education Reform Now. "It's been 25 years since the city has experienced a strike, and quite frankly, people get amnesia or somehow just don't know what that means for struggling families in our communities."
At several expressway overpasses, Parents 4 Teachers, a pro-CTU group held signs and banners saying "Quality schools for all kids" and "Parents stand with teachers" to drivers honking their horns.
"I think most parents really are in support of the teachers and we just need to convey that to CPS and the mayor," said parent Amy Green, 34, as she held a stack of flyers. "I feel like maybe when the parents do get involved, the mayor will listen."
For now, parents have been busy signing up children for the district's contingency plan which will open 144 school for half a day and 60 churches from 8:30 a.m. till 2 p.m. in the event of a strike. On Friday, schools CEO Jean-Claude Brizard asked Lewis to agree not to picket in front of schools that are open as part of the plan.
"Since students will stand to lose the most from a strike, I have deep concerns about the impact of forcing kids to walk through a picket line with their parents," Brizard said.
Asked about the letter, Lewis said the sites should be shut down, describing the plans to use central office employees and administrators to staff the locations as a "mess."
At Ogden International School of Chicago on Friday, students leaving for the day said their teachers had given them mixed messages on what to expect next week. Some told students they'd see them on Monday while others offered the opinion that the strike could last three to four days.
"Most teachers said you should act like you're coming to school, so do your homework," said Henry Allen, 12.
That offered little comfort to parents, many of whom said they are not sure what to expect.