Union Vice President Jesse Sharkey said the group endorsed Napolitano "understands the importance of strong schools."
"[Napolitano] has shown his support for teachers and students long before deciding to run for office, as a volunteer at Chicago Public Schools and by wearing his red shirt in solidarity with the Chicago Teachers Union during our 2012 strike," Sharkey said in a statement.
Napolitano said he was honored to have the endorsement of the teachers union.
"Strong neighborhood schools are the foundation of our community, and I share the Chicago Teachers Union’s vision to ensure that all students have access to a quality education," Napolitano said in a statement.
O'Connor, who is running for a second term, frequently voted with Mayor Rahm Emanuel on a host of issues. The union is supporting Cook County Commissioner Jesus "Chuy" Garcia in Tuesday's mayoral runoff.
O'Connor said she was proud of her record on schools, which included obtaining $100 million for 41st Ward schools and other "achievements virtually unheard of for a freshman member of the council."
Napolitano said he supports the teachers union's agenda, including a push to make the members of the Chicago Public Schools board elected rather than appointed by Emanuel.
The 41st Ward, which includes Edison Park, Norwood Park, O'Hare and Edgebrook, is home to a large population of city workers, police officers and firefighters.
The most recent poll of likely voters by Aldertrack in the 41st Ward found O'Connor had 53.6 percent of the vote to Napolitano's 46.4 percent, with a margin of error of 4 percentage points. The results predict a close race that will be decided by turnout, concluded polling firm Ogden and Fry.
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