Chicago Teachers Union President Karen Lewis is putting aside her mayoral ambitions while she battles a brain tumor, the Chicago Sun-Times reported Monday. The charismatic firebrand was set for a hotly anticipated standoff with Chicago mayor Rahm Emanuel in his bid for re-election. 

Her mayoral bid was an outgrowth of the political momentum Lewis, a former chemistry teacher, gained when she and the Chicago Teachers Union took on Emanuel in an historic 2012 citywide teachers strike. In that fight, Lewis and the union refocused a mainstream education reform conversation typically depicted as one between self-interested teachers unions and everyone else into a conversation about equity and children’s educational rights in a constrained, anti-labor climate. 

It’s little coincidence that their showdown happened in Chicago, President Obama’s hometown and a testing ground for the school-reform policies championed by Secretary of Education Arne Duncan and now executed by Rahm Emanuel. Among the most controversial of those policies has been school closures, which advocates argue disproportionately impact black and Latino students. Last year Emanuel shuttered 49 schools. Polls conducted by the Chicago Tribune in August show that voters have been siding with unions instead of Emanuel when it comes to handling schools.

Without Lewis in the race, Emanuel’s lost his most formidable opponent, the Chicago Tribune reported this morning.