CHICAGO – Teachers in the nation’s third-largest school district voted overwhelmingly to authorize the first strike in 25 years if their union and the city cannot reach a deal on a contract this summer – signaling just how badly the relationship between teachers and Chicago school officials has deteriorated, union officials said Monday.

Chicago Teachers Union President Karen Lewis announced the result of last week’s balloting – nearly 90 percent of its 26,502 members voted to authorize a strike – and called it “an indictment of the state of the relationship between the management of CPS and its largest labor force members.” State law requires 75 percent approval.

Teachers are upset that Mayor Rahm Emanuel canceled last year’s raise and that they’re being asked to work longer days without what they consider to be an adequate pay increase. Lewis said other key issues include class size and resources.