Updated: The Chicago Teachers Union said Tuesday night that city parents should begin preparing for a “short-term” strike. Read more here. 

With the clock ticking down toward a Chicago teachers strike and a recent poll showing public opinion favoring teachers, Mayor Lori Lightfoot has made a significant shift in her bargaining position, saying the city is ready to write staffing and class size promises into the union contract. 

The union demand to put staffing promises in writing has been a key sticking point in months of contract negotiations, with the union demanding that promises be contractually binding and the mayor saying she had put her intent to increase staffing in the city’s budget. 

“We have expressed a willingness to find solutions on these two core issues that would be written into the contract,” Lightfoot said during a press briefing Tuesday afternoon. 

However, just a few hours later, Chicago Teachers Union President Jesse Sharkey said that significant “gaps” remained between City Hall and union negotiators on several issues, including class size, that make a walkout all but unavoidable.

Sharkey also said Tuesday that the city and a separate union, Service Employees International Union Local 73, representing school support staff and park district workers, had not yet reached a deal.

“Three bargaining units, two unions, one mayor, and no deals,” Sharkey said. “Something has got to shift there.”

Related: Chicago is the latest front for ‘common good’ bargaining

A survey conducted by ABC7 and the Chicago Sun-Times of more than 600 Chicagoans and released Monday night found that 49% would support a teachers strike, with 38% opposed. 

What an agreement could look like remains unclear. In negotiations over the weekend, the union suggested a compromise on its demands that Chicago hire more nurses, social workers, and special education case managers, by agreeing to staff the neediest schools first. But on Monday night union officials said they had not yet reached an agreement on the proposal. 

Bargaining will continue Wednesday. 

The union’s representative 700-member House of Delegates must approve any proposed contract agreement before a strike is averted. Leadership has called a House of Delegates meeting for Wednesday evening, when members will vote yes or no on the city’s latest offer. 

Meanwhile, parents all over the city received robo calls from the district updating them about strike plans for students. 

Schools chief Janice Jackson said contingency plans are in place for the 300,000 Chicago students who could be displaced by a strike. Separately, the city has been bargaining with a union representing school support staff and park district workers, who have also authorized members  to walk out with teachers Thursday. 

District officials said they will cancel classes, tutoring and after-school activities, but plans to keep schools open to offer child care and free meals for students. Students with special education or nursing needs will not get services, but the district is working on lining up some contract nurses, Jackson said Tuesday.