In Chicago Mayor’s Race, a Former Teacher Rises With Union Support

Brandon Johnson had a problem. In a crowded Chicago mayoral race full of established liberal politicians — a sitting congressman, the incumbent mayor, two City Council members — many voters had never heard of Mr. Johnson, a county commissioner from the West Side.

But he had something those other contenders did not: the Chicago Teachers Union.

Loved and loathed, the teachers’ union has emerged over the last dozen years as a defining voice on Chicago’s political left, putting forth a progressive vision for the city that extends well beyond its...