The Chicago Teachers Union is shaping up to be Mayor Rahm Emanuel’s toughest opponent. The former Obama White House chief of staff has trimmed his Democratic National Convention visit with the very real threat of the city’s public school teachers going out on strike come Monday.

When asked about the strike threat Tuesday, the usually eager-to-engage mayor instead continued greeting children arriving at Shields Middle School. Today was the first day of school for some 400,000 school-aged kids in Chicago.

Emanuel will arrive in Charlotte late Tuesday to make a speech at the convention. But instead of staying through President Obama’s speech on Thursday, Mayor Emanuel will return Wednesday night. The mayor’s spokeswoman, Sarah Hamilton, said the Obama campaign asked Emanuel to host a speech-watch party for campaign staffers in Chicago.

But the schedule change allows Emanuel to be home while a crisis may be brewing. The biggest issue contributing to the strike threat is a longer school day and school year. Both were high on the list of campaign promises for Emanuel when he was campaigning to be Chicago's mayor. Once elected, Emanuel worked methodically to make those promises a reality.

In a statement Tuesday, Emanuel praised the longer school day he engineered.

“Chicago’s students now have a full school day and a full school year that measures up to their full potential," the statement read. "Through hard work and cooperation, we have finally put an end to saddling our children with the shortest school day and shortest school year of any major city in the nation; we have put an end to shortchanging Chicago’s students.”

The Chicago Teachers Union sees it differently. It sees the changes in length in school day and year, new teacher evaluation methods and a new curriculum as an effort to goad the teachers into a strike. Last week, a strike notice was delivered to the Illinois Department of Education.

Then on Labor Day, in Daley Plaza across the street from City Hall, an estimated 10,000 union members, reportedly including the American Federation of State, County and Municipal Employees (AFSCME), the Service Employees International Union (SEIU) and Chicago police and firefighter unions, railed against Emanuel and his handling of public schools. Teachers Union President Karen Lewis drew cheers calling Emanuel “a liar and a bully”.

With the school year now underway, Jean-Claude Brizard, CEO of the public school system remains optimistic, telling one local news outlet, “We’re going to get this thing done so no one is going to walk off (the job).”

But bargaining sessions are continuing in private. Whatever wiggle-room the Chicago public schools may have had, may be long gone. To lessen a $600-million projected budget deficit for the current year, the school system wiped out all of its financial reserves, some $432 million. So with the budget already in the red to start the school year, it’s uncertain what Chicago could offer its 26,000 teachers union members, who are readying for a strike on Monday.