During campaign season, the Chicago Teachers Union made it pretty clear who their favored mayoral candidate was—and it wasn't Lori Lightfoot. Anyone who hoped the summer season would warm up the chilly relations between that contest's eventual victor and the city's disgruntled public school teachers just got a reality check.

As Crain's political columnist Greg Hinz was first to report July 11, the CTU issued an explicit strike threat—an our-way-or-the-highway declaration that followed weeks of increasingly sharpening rhetoric.

In an email, CTU President Jesse Sharkey repeated recent complaints that the new mayor has not followed through on campaign promises to boost early childhood education, neighborhood schools and the like. "Those campaign promises mirrored the equity agenda of our movement," said Sharkey.

Then Sharkey made his threat.

CTU teachers and other staffers returning to school next month "deserve a fair contract with equal pay, adequate staffing, class size limits and social justice for their students," he wrote. "Candidate Lightfoot has vowed there will be no teachers strike on her watch. Mayor Lightfoot has a month to make good on that and her campaign promises."

Negotiations between the Board of Education and the teachers are, as of this writing, ongoing. Chicago Public Schools recently floated its first financial offer, an across-the-board 14 percent cost-of-living hike over five years. But the union complains the offer is really only 12.5 percent after increased employee payments for health insurance are factored in.

So what does the union want? Five percent annual raises—which would amount to a pay increase of about 27 percent after compounding if granted over the same five-year period. Board sources tell Crain's the union's proposal would cost taxpayers an additional $2.5 billion.

Now the board's initial offer as well as the union's bellicose reaction to it may be nothing more than the sort of preliminary posturing that's common in the professional wrestling ring, in crowded DMV waiting rooms and in union negotiations. And if Sharkey's talk winds up being just that—talk—that's fine. He's made his members happy. But there's a chance it's more than that: an effort to intimidate a new mayor with tactics that echo the run-up to the strike that marked her predecessor's early tenure in 2012.

We can only hope the people who are actually sitting at the table hammering out a deal are more willing than Sharkey seems to be to reckon with the financial hardships facing not just CPS—which has only recently returned to something like fiscal stability—but the entire city of Chicago as well as the state. No small share of the pension obligations that threaten to overwhelm us all is owed to teachers. Chicago taxpayers, many of whom expect nothing like a guaranteed 27 percent raise, have been asked to dig deep to pay down those obligations, and may be asked to pay still more. Against that backdrop, where does Sharkey propose finding an extra $2.5 billion? At what point will the teachers recognize that driving their employers to insolvency is not in their best interest?

Or, put another way: When will the teachers ever learn?