The Chicago Teachers Union, school support staff and Park District workers will all go on strike together on Oct. 17 if they can’t reach contract deals by then.

The joint announcement late Wednesday by the three labor groups sets up the prospect of nearly 35,000 public employees in Chicago walking off the job at the same time.

It also means that the 360,000 children who attend Chicago Public Schools will be out of class indefinitely if CTU and the city fail to settle their differences by then.

But Mayor Lori Lightfoot and CPS top brass announced late Wednesday that all schools will remain open during any walkout by teachers and staff.

“In the event of a strike, all CPS school buildings will remain open during their normal school hours to ensure students have a safe and welcoming place to spend the day and warm meals to eat,” the mayor and CPS chief Janice Jackson said in a joint announcement.

They stressed they will also do “everything in our power to reach a fair deal that prevents a strike.”

In revealing the plan for a potential three-way walkout, CTU President Jesse Sharkey said: “The schools don’t work without all of us, and our unity is our strength.”

He added: “We prefer to reach a contract settlement without a strike. … But I want no one in the city of Chicago to doubt our resolve. We mean to improve the conditions in our schools. We mean to achieve a fair contract.”

The CTU represents 25,000 teachers employed by one of the country’s largest school systems. But the threefold announcement Wednesday means that another 7,000 CPS workers — security guards, bus aides, special education classroom assistants and custodians — and about 2,500 employees of the Chicago Park District could also all end up on picket lines together.

Each of the three labor groups had already authorized separate strikes but had not set strike dates until Wednesday. The teachers could have decided to walk out as early as Monday, but decided to try to harness the collective power of a larger labor coalition.

“We are united with CTU and with workers standing up for themselves,” said Dian Palmer, president of Service Employees International Union Local 73, which represents both the parks workers and CPS support staff.

While the later date gives the teachers union and the city more time to settle their differences, it’s clear the two sides remain far apart, with both accusing the other of stalled responses to demands and offers.

Prior to Wednesday’s announcement, Lightfoot and officials in her administration again called on the teachers union to get a deal done without the disruption a strike would cause.

“I’m not naive about the complexities here. But I remain hopeful that if they come to the table in good faith, if they respond to our written offers that are out there, we’ll get a deal done,” the mayor said Wednesday. “And we should, because it’s in the best interests of our children and their families to get it done.”

The city’s immediate response to blunt the impact of a possible strike was its announcement that all school buildings in the system would remain open during any teacher and staff walkout. Regular instruction would not take place, but principals and non-union staff would be on hand “to greet students and ensure they have access to engaging activities,” CPS said in a letter to families. Transportation services will not be available, though.

Charter and contract schools will maintain regular operations, officials said.

SEIU Local 73 Executive Vice President Jeffrey Howard said acknowledged one reason the three units decided to set a joint strike date was because, in past teachers strikes, many parents send their children to park facilities.

This joint action, he said, “is about taking away that avenue and forcing (employers) to negotiate in good faith. … They want to pit workers against each other.”

Lightfoot said she wasn’t surprised to be facing all this labor pressure so soon after taking office. The writing was on the wall with the number of high-profile contract negotiations she walked into, she said.

Lightfoot’s administration and the Board of Education she selected, however, are facing a union whose rank-and-file members have indicated strong support for a walkout.

A huge majority of CTU members — 94% — voted in favor last week of authorizing a strike.

The teachers union has stressed their demands not just on pay and benefits but also a number of staffing issues, including more librarians, school nurses and social workers.

Leading up to the delegates’ meeting, Sharkey said Wednesday afternoon that the union’s “strategy is obvious. We’re trying to get the board to talk about these staffing issues.”

“I just think it, if the mayor’s point is we are not responding, let’s be clear: We are spending a lot of time at the table, putting a lot of energy into it,” he said.

But the city’s assertion that the union has been unresponsive to its offers was reiterated late Tuesday, with one of Lightfoot’s top negotiators expressing frustration that the CTU hadn’t yet provided the city a formal, written counter-proposal to its contract offers.

The union hasn’t provided Lightfoot’s negotiators with anything in writing since Sept. 5, said Michael Frisch, a Lightfoot senior advisor who represents her at the bargaining table.

“As you know, we gave them a comprehensive offer on Friday. Again today we received nothing back in writing which was disappointing because as you know, we are working hard from our side to present comprehensive offers to the CTU,” Frisch said. “We are very interested in getting a deal, reaching an agreement quickly, and avoiding a strike that would be disruptive for our students and families. We’re not seeing that same level of urgency across the table.”

Sharkey countered Wednesday: “Our response to a number of the proposals is no. It’s not a very long response, but it’s a response. We are not interested in giving back rights that we won over the last contracts.”

The union hasn’t provided formal responses to the administration’s proposals but Frisch said they’ve brought clinicians, counselors and teachers to the table so the administration can hear their viewpoints. He called that a “useful discussion.”

“It’s helpful, but what we really need at this point is a comprehensive package offer for what it will take to get this resolved,” Frisch said. “We haven’t seen that yet.”

Frisch stressed that, on Friday, CPS sweetened its offer by promising that a number of workers would immediately get raises under the deal. Health service nurses would immediately get an average 14%, while school clerks would be bumped up by at least 8% while teacher assistants would get 9% increases on average, Frisch said.

The city has also offered to keep health insurance costs for teachers unchanged for three years of a five-year pact and then raise them by less 0.75% over the last two years, Frisch said.

“This is a big deal, and I think it’s deserving of a serious response from the CTU,” Frisch said. “We’re still waiting for them.”

The city has also offered teachers 16% raises over a five-year contract, not including separate raises teachers received for each year of service. The CTU has asked for 15% over three years.

At CTU headquarters Wednesday after the joint strike announcement was made, labor leaders’ comments focused mainly their groups’ unity. But they also took jabs at city officials.

“I’ve a question for the mayor and the people who run the parks and schools: Do they feel guilty when they pay workers less than $20,000 a year?” Palmer, the SEIU local president, said.

Tribune reporter John Byrne contributed.

hleone@chicagotribune.com

gpratt@chicagotribune.com