CHICAGO — In some of this city’s predominantly black neighborhoods on the South Side, Rahm Emanuel was not well known when he swept into the mayor’s office in 2011. After all, he had been a congressman from the other, mostly white side of town. But most voters knew that President Obama, himself a South Sider, had trusted him to be White House chief of staff, and that backing went a long way. Mr. Emanuel won every majority black ward in the city.

Fast-forward four years and Chicago has gotten to know Mr. Emanuel. He oversaw the closing of nearly 50 public schools, and many of the schools were in minority neighborhoods on the South and West Sides. He clashed with the Chicago Teachers Union, which went on strike for the first time in a quarter-century. He presided through an increase in killings that included more than 500 homicides in one year, most of which took place on the South and West Sides.

Seeking a second term, Mr. Emanuel is seen as the front-runner beside four lesser-financed, lesser-known candidates. Yet as he tries to win re-election next week, an essential worry for his campaign is how well he will do with black residents, who make up about a third of the city’s population of 2.7 million. To avoid a runoff in April, Mr. Emanuel must win at least one vote more than 50 percent on Tuesday — an outcome that is threatened by the possibility of a weak showing from alienated black voters.

“It all seems to be just for the people who have money, not for the middle-income or the lower-income,” said Mary Anderson, a nursing educator from the South Side’s Chatham neighborhood. She said she supported Mr. Emanuel in 2011 but had yet to decide this time.