CHICAGO (CBS/AP) — Chicago teachers were voting Tuesday on whether to authorize a strike next month as contract negotiations continue in the nation’s third-largest school district.

Voting members of the Chicago Teachers Union started casting ballots Tuesday.

Democratic presidential candidate Bernie Sanders joined a teacher rally Tuesday evening.

“You know, every problem in society – hunger, domestic violence, poverty – it walks into your doors, doesn’t it. You see it every day, and at a time when we in the wealthiest country in the history of the world have the highest rates of childhood poverty of almost any major country on earth, you are demanding and I am demanding a change in national priorities,” Sanders told the teachers.

The voting will continue until at least Thursday. To strike, 75% must vote in favor. The earliest a strike could take place is Oct. 7.

Educators and Chicago Public Schools officials have been negotiating pay and staffing shortages, among other issues.

Union members say they remain far apart. District officials say their latest offer is strong.

CTU wants a 15% raise over a three-year contract. The district’s latest offer is a 16% total raise over a five-year contract.

Meanwhile last month, the Chicago Teachers Union rejected an independent fact finder’s contract recommendation, setting up a possible strike if they cannot reach a deal with CPS.

The recommendation that the union rejected involved an offer to boost pay checks – specifically a 16 percent pay increase over five years. Teachers are asking for more than money.

They want smaller class sizes, and more special education teachers, social workers, and librarians. Four or five years ago, CPS had 400 librarians, and next school year the district will have only 108, according to CTU President Jesse Sharkey.

Mayor Lori Lightfoot said Tuesday that she had heard the unions’ concerns about a lack of special ed teachers, counselors, nurses, and other support staff, and added, “That’s why we have baked those into the budget for this year.”

“There’s absolutely no reason why we shouldn’t get a deal done, and as I said last week, we will double, triple track, seven days a week, to get it done,” Lightfoot said.

If a strike did go ahead, it would be the first since 2012.

(© Copyright 2019 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)