Teachers at 15 Acero schools overwhelmingly voted Tuesday evening to authorize a strike, setting the stage for the first walkout in the nation by teachers at a charter network.

With a 96 percent turnout of the estimated 500 union-represented Acero Teachers, 98 percent of members voted to grant a strike authorization. The teachers union can now announce a strike date if contract negotiations reach an impasse, according to the Chicago Alliance of Charter Teachers and Staff (ChiACTS).

Acero, formerly named UNO, is the largest unionized charter-school operator in Chicago Public Schools. Its contract with teachers expired Aug. 2 and was extended until Oct. 3. But talks have been stalled, union officials said.

If teachers do walk out, it could be the country’s first charter school strike, union leaders said.

At issue in the contract negotiations are higher pay, increased diversity among teaching staff in majority Latino schools, smaller class sizes, better special education services and teacher evaluations.

Chicago International Charter Schools teachers will also take a strike authorization vote Friday.