In a stunning upset, Chicago’s Democratic machine suffered a big defeat on Tuesday night, as Cook County Assessor Joseph Berrios was defeated in a Democratic primary by insurgent Fritz Kaegi. As of this writing, Kaegi had 45 percent of the vote to Berrios’s 34 percent. A third candidate, Andrea Raila, had 21 percent. Berrios has conceded.

The Chicago Tribune marked the victory as a watershed moment for the activists who had backed Bernie Sanders’s presidential bid in 2016. “The race for assessor, typically a quiet, down-ballot affair, had a much higher profile this year because it became a test of the ability of progressives in the wake of U.S. Sen. Bernie Sanders’s presidential campaign to take on establishment Democrats and win,” it noted.

Several other down-ballot races also featured strong performances by upstart candidates. Daniel J. Burke, who has been in the state House since 1990, was defeated in his Democratic primary by Aaron Ortiz. Ortiz is an educator and high school counselor at a public school in the Chicago area, and ran on establishing tuition-free undergraduate college, legalizing marijuana, single-payer health care, and ending cash bail. In the 4th House District, Delia Ramirez won her primary by a large margin. Brandon Johnson, a Chicago teachers union organizer, edged out machine-aligned Richard Boykin in a startling upset.

All four candidates were backed by United Working Families, a coalition group formed to challenge the corporate dominance of politics, and linked with the Working Families Party.

County assessor, meanwhile, may seem like a fairly obscure position. But municipal assessors are charged with valuing properties and thus, determining property tax responsibilities. Cook County machine politicians have been able to use control of the assessor office to leverage contributions and other benefits. For years, Cook County has failed to accurately value homes, which leads to a disproportionate share of taxes falling on poorer residents, many of whom are African-American and Latino. These residents lack — or lacked — the political power to change the system. Until now.

As was highlighted in a profile of him in The Intercept during the campaign, Kaegi is hardly your image of a rebellious candidate. A Stanford MBA graduate and former mutual fund manager, he fits the profile of many of the upper-crust Democrats that currently run Chicago’s governing machine.

But Kaegi’s campaign quickly built a progressive base by taking aim at Cook County’s unfair tax assessments. Our Revolution Illinois, after some initial skepticism given his professional background, became a convert and made him a marquee candidate, backing his run for office while simultaneously bird-dogging Berrios’s office to pressure him to change the system. “We’ve made a conscious decision to make this a top priority,” Our Revolution chair Clem Balanoff told The Intercept. “If you look at the [most loyal] voters of the Democratic Party, they’re the ones who are getting screwed the most.”

Berrios’s campaign was backed by a who’s who of local Democratic Party bigwigs, from retiring Rep. Luis Gutiérrez to the powerful state House Speaker Mike Madigan. He also received millions of dollars from property tax appeal lawyers — the same lawyers the county’s poorest residents are forced to turn to in order to have their properties fairly assessed.

The down-ballot wins, said United Working Families Executive Director Emma Tai, say as much for this moment as they do for the future of the movement, which is finally rebuilding a bench of talent. “The United Working slate was comprised of young people of color who were first-time candidates. They took on big-money interests and the Democratic machine and they won,” she said. “These victories didn’t all happen just tonight. They come from long-term organizing that goes back to the 2015 elections that helped spawn robust independent political organization around Chicago. We’re stand ready to take on the corporate Democrats who have let incarceration, violence, gentrification, and unemployment ravage our communities. And tonight’s results show that the voters are with us.”