Here’s the challenge for the next mayor of Chicago: Bring back the middle class.

We don’t mean save the middle class. That would mean Chicago has one. We mean bring it back.

Because the stunning truth about our city is that the middle class is almost gone.

A city without a middle class is no place to be. It is a ladder with a few rungs at the top and a few at the bottom — and none in the middle. It is a place where the social mobility essential to a democracy — the ability to climb the ladder of success — becomes prohibitively difficult, if even possible.

The next mayor of Chicago may argue that the biggest challenge facing our city is getting its finances in order, or improving its schools, or curbing violent crime. But all those challenges must be met in ways that pull the whole city along — that create opportunities for the poor to move up and for the last of the middle class to stick around.

Or what’s the point?

The alarming disappearance of Chicago’s middle class has been documented by researchers at the University of Illinois at Chicago, and recently was summed up and analyzed by WBEZ, the public radio station.

A set of maps by the university’s Nathalie P. Voorhees Center for Neighborhood and Community Improvement reveals that fully half of Chicago’s 797 census tracts were middle income in 1970, but only 16 percent are today. Most of those formerly middle-income census tracts now are low or very low income.

Meanwhile, in a measure of how polarized Chicago has become, just 8 percent of the city’s census tracts in 1970 were considered high or very high income, but today 20 percent are high or very high income.

The Voorhees Center researchers define a census tract as “middle class” if the average per capita income is between 80 and 120 percent of the metropolitan-wide average for that year.

As WBEZ reports, the economic polarization of Chicago was driven early on by middle-class white families leaving town as entire neighborhoods flipped racially. The folks who took their places, often Latino or African-American, generally had lower incomes.

That trend has been followed, especially on the North Side, by gentrification, as highly educated younger people with excellent jobs have displaced middle-income residents.

Driving the upheaval and displacement has been the nature of America’s changing economy, which increasingly consists of high-paying jobs in new tech fields, such as writing software programs, and low-paying jobs in the service sector, such as making two-shot soy lattes for those software programmers.

What Chicago and much of the nation have lost is the kind of work that supports a secure middle-class life, most notably in classic manufacturing, and often union jobs.

Pretty much everything gets turned upside down.

“Conditions now force many middle-income families to either leave the city to find housing they can afford, or to move into lower-income communities,” Janet Smith, co-director of the Voorhees Center, told us. “Leaving the city often means longer commutes to work, often driving rather than using public transportation. Moving to lower-cost housing areas helps to further fuel gentrification — intentional or not — which can then continue to exacerbate the divide.”

Smith points out that the shrinking of Chicago’s middle class brings with it another loss: Families with children. Today, only 25 percent of Chicago households include kids under the age of 18.

“Politicians always talk about families,” Smith said, “but if this is a city for families we will need to look at housing affordability and school policy to really support them. So they can thrive and be part of building a better Chicago.”

As the next mayor of Chicago takes on the city’s endless challenges — unemployment and underemployment, shaky finances, inadequate schools, crime and all the rest — let this be a guiding principle:

Chicago must regrow its middle class.

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