CHICAGO – The Chicago Teachers Union and district officials said they will negotiate through the weekend, if necessary, to try to avoid a strike Monday.

Teachers are ready to walk off the job for the first time in 25 years over issues that include pay raises, job security and teacher evaluations.

There is a lot riding on this contract for Mayor Rahm Emanuel, who has made school improvement a central theme of his administration, and for the teachers union, which feels its ability to protect teachers and give them a voice in education is under attack.

Both sides said Friday that they were optimistic that a contract could be settled without a strike, although a union spokeswoman said they remained “far apart” on issues such as job security and pay.

Union President Karen Lewis said Friday night that she was disappointed with the latest offer from the school district, saying it didn’t answer members’ concerns and “we are here to negotiate for better schools in Chicago.” She said negotiations would resume Saturday.

The district offered a cost-of-living raise of 2 percent a year for four years, which the union said was unacceptable — especially after Emanuel last year canceled a previously negotiated 4 percent raise, citing budget problems. A school district spokeswoman has said that raise will not be made up and the district will not address it in negotiations. The union has lowered the amount it’s asking for, but has not said what its counterproposal is now. Only weeks ago, it sought a 19 percent raise in the first year of the contract.

The union also is concerned about raises based on teacher experience and education. It has said the district agreed to retain language allowing raises based on experience, called step increases, but it would not actually pay the money now. Keeping the language in the contract, though, could be important for teachers in future negotiations.

The union and school district began negotiating in November on a contract that was to expire seven months later, in June. Things heated up in May, when teachers picketed over a lack of progress on talks. Then in June, 90 percent of teachers voted to authorize a strike if a contract wasn’t reached over the summer.

It all began when Emanuel last year asked the union to re-open its existing contract and accept 2 percent pay raises in exchange for lengthening the day by 90 minutes.

The union refused, noting he’d already rescinded 4 percent raises over the summer. Emanuel, who had won legislative approval to lengthen the school day, then tried to go around the union by asking individual schools to waive the contract and add 90 minutes to the day — until the Illinois Educational Labor Relations Board told him he couldn’t.