CHICAGO – Chicago Mayor Lori Lightfoot swept into office this spring declaring it “a new day” for the nation’s third-largest city. She pledged to change the way City Hall operated under Rahm Emanuel and for decades before him, invest in poor neighborhoods, improve schools and address the city’s deeply troubled finances. But with teachers in Chicago Public Schools hitting the picket lines this week, Lightfoot finds herself facing many of the same challenges as her predecessor, the former White House chief of staff whose years of conflict with the Chicago Teachers Union included a seven-day strike in 2012. About 25,000 teachers and staff were on strike for the second day Friday, after months of negotiations ended without a new contract between the union and CPS.