CHICAGO (WLS) -- Members of the Chicago Teachers Union began voting on whether to authorize a strike Tuesday, and Democratic presidential candidate Bernie Sanders is expected to join them at an evening rally.The Vermont senator is set to join CTU members for a rally at union headquarters in the West Town neighborhood at 7 p.m."Well it's a big deal obviously, I mean national Democratic Party candidate, going to be putting a lot of media attention on the fact that teachers in Chicago are fighting for staffing, are fighting for class size, fighting for fair pay and benefits," said CTU President Jesse Sharkey."I'm hopeful that causes the leadership here in Chicago to take another look at their refusal to bargain in good faith for the schools Chicago students deserve," CTU Vice President Stacy Davis Gates said of Sanders' visit.The Chicago Teachers Union has also recently received supportive tweets from other Democratic presidential hopefuls Elizabeth Warren and Joe Biden.Members of several other unions will also be at the rally, including support staff at Chicago Public Schools who are represented by Service Employees International Union Local 73. Striking General Motors workers are also expected to join the group.The teachers union said talks with the city have been frustrating. The city has offered teachers and support staff 16 percent raises over five years, but union officials said they are focused on more than just money."We wanna have class size limits. We've got schools with kindergarten, first grade classes with classes as high as 40 students," Sharkey said."Unless these basic and minimum standards are being met, we are not going to be able to meet our students' needs," said CPS teacher Epice Holda.But the city maintains those issues are not part of collective bargaining, nor can teachers legally strike over classroom sizes, nurses and librarians. Union members can walk off the job over pay and benefits.The teachers will begin voting on whether to authorize a strike starting Tuesday and ending at 6 p.m. Thursday, when the results will be expected.At least 75 percent of the teachers need to support a strike in order for it to pass. The earliest a strike could happen is October 7."We should be able to reach an agreement," Sharkey said. "We have a mayor who campaigned on the idea of basic educational supports, on the idea of equity, supporting neighborhood schools. Those are all things we agree with strongly and we thinks she'll open our contract proposals."Another bargaining session was held Tuesday in the city. Mayor Lori Lightfoot has said she would come to the bargaining table if necessary, but the CTU says it hasn't been necessary yet.