(CBS) – Chicago Teachers Union President Karen Lewis, who is undergoing treatment for a brain tumor, is seen in a video posted online Friday endorsing mayoral candidate Jesus Garcia.

The video, posted on YouTube, was expected to be played Friday evening at an organized labor event.

Wearing a scarf on her head, a smiling Lewis explains her decision not to run for mayor and goes on to endorse Garcia, a Cook County commissioner, for mayor.

Lewis herself had considered running for mayor in hopes of unseating incumbent Rahm Emanuel, until she was abruptly diagnosed with a brain tumor and underwent surgery.

Among the other mayoral candidates is 2nd Ward Ald. Bob Fioretti.

Garcia was grateful for the nod from Lewis.

"Karen Lewis -- and you know it better than I do -- is one of the best warriors that Chicago has experienced in over a generation," Garcia said in response to the endorsement.

Fioretti says he intends to fight for the endorsement of the union rank-and-file.

CTU Vice President Jesse Sharkey has assumed Lewis's duties while she undergoes treament.