Chicago Teachers Union delegates from each school decided Wednesday evening that their union will hold a strike authorization in late September if a deal cannot be reached before then. The decision was unanimous.

The voting will take place between Sept. 24 and 26.

Teachers and other staff could walk out immediately after the vote, but the union is now saying the earliest it would strike is Oct. 7.

According to a controversial state law, 75 percent of the union’s total membership of 25,000 teachers, clinicians, paraprofessionals and retirees must authorize a strike in order for the leadership to call one. This bar, set in a 2011 state law, was intended by those advocating for it to prevent a strike. But in 2012 and 2016, strikes were easily authorized.

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The CTU contract with Chicago Public Schools expired at the end of June and the two sides have been in intense negotiations all summer. Both sides say progress has been made recently. Chicago Mayor Lori Lightfoot this week said she’s confident a deal can be reached.

But union leaders say they are still not satisfied with how the school district has responded to many of their demands. Among other things, the union is pushing for language in the contract that promises more nurses, social workers and counselors. The union also wants lower class size limits and wants them to be enforceable. Now they are only advisory.

Lightfoot and school district leadership say they believe in much of what the union is demanding. But they are resistant to putting commitments in the contract. Per state law, teacher strikes in Chicago can only be called over compensation and related issues.

Lightfoot and CPS CEO Janice Jackson say the two sides are close on salary and benefits. The city’s last offer was a 16% pay raise over five years, up from 14%.

But union leadership can hold out on agreeing on salary and benefits until other demands have been met. Also, they have said they are not happy with the compensation offered to paraprofessionals and other low wage workers they represent.

Sarah Karp covers education for WBEZ. Follow her on Twitter at @WBEZeducation and @sskedreporter.