City Council members spoke out against the new $99-million contract on Tuesday.
Under the contract, 300 unionized custodians would be replaced with lower-paid non-union workers.
Emanuel says the contract will help save money, but critics argue that there is more on the table.
"We all want the bottom line to be fiscally responsible, but we don't want to balance that on the backs of our workers," said Ald. Roderick Sawyer.
The current contract will force O'Hare janitors out of their jobs by Friday.
The Office of the Mayor issued this statement on Tuesday: >br>"Mayor Emanuel always puts Middle Class families first. Whether it is cracking down on pay-day lenders that prey on families living paycheck-to-paycheck; expanding opportunities through new innovative partnerships with labor; or refusing to increase property and sales taxes, Mayor Emanuel has proven to be a passionate fighter for the hardworking middle class families of Chicago."