Unlike last year’s red-state teacher uprisings, Chicago’s education workers walked out Thursday morning against a Democrat: newly elected mayor Lori Lightfoot. The walkout reveals a key problem facing Democrats and their crowded field of 2020 presidential candidates: It’s not easy to square the circle between promises on the campaign trail and policy once in office.

In some ways, this is a novel dilemma for a party that only a few years ago was openly championing a billionaire-funded school privatization agenda. But the political tides have turned since the Chicago Teachers Union (CTU) struck against Democratic Mayor Rahm Emanuel in September 2012.

Though President Trump’s secretary of education, Betsy DeVos, is partly responsible for peeling off education reform’s formerly progressive veneer, the strikers themselves deserve most of the credit for turning the tide. Ever since West Virginia educators walked out in February 2018, Democratic politicians have increasingly shied away from their previous advocacy of charter schools, austerity, and punitive teacher-accountability schemes. Even Cory Booker, one of education privatization’s most fervent backers, has recently rebranded himself as a friend of public schools and labor unions.

It’s a sign of the times that Lightfoot was elected earlier this year on a progressive education platform and a pledge to break the school-to-prison pipeline through increased funding. Yet, within months of taking office, Lightfoot immediately began to backtrack. “The candidate that people in this city voted for said that she agreed with them that our schools were a priority and we had to resource them,” observes CTU Vice President Stacy Davis Gates in reference to Lightfoot. “The mayor is saying something completely different.”

Despite an influx of over a billion dollars in increased yearly school funding for Chicago since 2016, Lightfoot and her negotiators are now pleading poverty in response to demands for smaller classes, a nurse and counselor in every school, and affordable housing support for both teachers and homeless students. After months of failed negotiations, 32,500 teachers and support staff walked out on Thursday morning over these issues.

Faced with entrenched political machines and the power of big business, it’s not surprising that progressive campaign promises don’t translate into policy for many Democratic officials. To buck such powerful interests means taking a considerable personal risk and leaning on disruptive collective action in the streets. Without such a movement, even elected politicians with the best of intentions can only do so much.

There’s a simple lesson here for all educators and voters eyeing the Democratic presidential contenders: Electoral promises are not enough. Without mass action, and the election of more class struggle candidates, the CTU’s vision of the “schools our students deserve” will likely remain a mirage in Chicago and across the United States.

But if Lightfoot’s evolution foreshadows what we might expect from a President Biden or Buttigieg, Chicago also offers a glimpse of what a Sanders presidency could look like. The city’s six recently elected socialist City Council members have not only championed the city’s educators against the mayor; they have also used their political platform and infrastructure to promote the educators’ struggles. Socialist aldermen Carlos Ramirez-Rosa and Rossana Rodriguez have co-organized community town halls with the CTU, and were on the picket lines at Avondale-Logandale Elementary Thursday morning. “This is what being a democratic socialist in office is all about,” Rosa says. “It means supporting organizers to win their demands, it means helping working people take their destinies into their own hands.”

Chicago is a microcosm of the deepening nationwide battle between the Democratic Party’s corporate-funded mainstream and an insurgent Left. Battle lines have crystallized around health care, student debt, and climate justice, but conflicts on the ground have been particularly acute in public education. An unprecedented upsurge in teacher strikes has made business as usual increasingly difficult for establishment politicians all across the political spectrum.

It’s a positive development that the educators’ revolt initiated by CTU in 2012 has made Democrats change their tune on public schools. But it will take many more strikes, and the election of many more socialists, to fundamentally transform our country’s educational priorities. The Democratic Party establishment won’t save public education. Chicago shows that we can only rely on ourselves.