Progressive challenger Cori Bush defeated Rep. William Lacy Clay in the Democratic primary for Missouris 1st Congressional District on Tuesday, adding to the activist lefts winning streak. The primary win in one of the most Democratic House seats in the country assures Bushs spot in the next Congress.

In remarks to supporters late Tuesday night, Bush characterized her win as the culmination of activist work that began when a white police officer killed Black teenager Michael Brown in Ferguson, Missouri, in August 2014.

It is historic that this year of all years, were sending a Black, working-class single mother all the way to the halls of Congress! she said over cheers and chants of good trouble.

Today, the people of St. Louis made a decision from all corners of Missouris 1st District, our communities have embraced a bold, fearless vision of real change where regular, everyday people like us can feel it. Today, the people won.

Bushs success follows an attempt to unseat Clay in 2018, when he won by nearly 20 percentage points .

But this cycle, with more endorsements, cash and name recognition a star turn in the Netflix documentary Knock Down the House helped Bush appears to have caught Clay by surprise. She outspent him on the TV airwaves in the final two weeks of the campaign.

Its a seismic shift in St. Louis politics, said Jeff Smith, a former Missouri state senator who now runs the Missouri Workforce Housing Association. Clay raised very little for an incumbent facing a serious challenge, and he paid the price.

Bushs victory is likely to have far-reaching effects within the Democratic Party as more incumbents wonder whether they too are vulnerable to a primary challenge from their left. Justice Democrats , the left-wing group that recruited Bush for her first run in 2017, has now ousted three House incumbents this election cycle with the possibility of one more in the coming weeks.

Perhaps more important, Clays loss marks the first time that Justice Democrats has knocked off a member of the Congressional Black Caucus. The CBC, filled as it is with moderate incumbents in solid blue seats, has been especially wary of Justice Democrats, arguing that the group represents a privileged, white fringe. Bushs win both validates the CBCs fears and complicates Black lawmakers simplistic characterization of the movement that drove her victory.

This is a huge upset and another groundbreaking win for our movement against a corporate-backed political dynasty, Alexandra Rojas, executive director of Justice Democrats, said in a statement noting that Clay is the fifth candidate unseated by one of the groups challengers since 2018. She organized a movement through pepper spray and rioting police in the streets of Ferguson. Her tenacity and unbreakable pursuit of justice is desperately needed in Congress today.

The outcome in Missouris 1st District bears many of the same David vs. Goliath dynamics of other major progressive upsets in the past two years.

Clay, who has represented the St. Louis area in the House since 2001, is a descendant of regional royalty. His father, Bill Clay , a labor organizer and civil rights leader, held the seat from 1969 to 2001.

Bush, by contrast, is a nurse, ordained minister and single mother of two who has experienced her share of economic hardships.

In an attack ad , Clay sought to make an issue out of Bushs failure to pay state taxes in a timely fashion, which resulted in her nursing license being temporarily revoked. Clays TV spot also hit her for using campaign funds to pay herself a salary since April, a practice that is increasingly common among non-rich candidates.

But Bush chalked up her tax debts to the financial difficulties she faced as a working-class mom paying down student debt, effectively turning the charge into a populist selling point.

In a TV ad of her own, in which Bush features images of herself marching in anti-racism protests, she casts herself as the change agent that a district with a substantial poverty rate needs.

Lacy Clay hasnt risen up to meet this moment, she says in the ad. Hes presided over 20 years of decline.

Bush ran in the mold of other left-wing House members and soon-to-be House members. She is a proponent of Medicare for All, student debt cancellation, the Green New Deal initiative and national rent control. Like Jamaal Bowman, the Bronx middle school principal who unseated Rep. Eliot Engel (D-N.Y.) in the June primary and subsequently endorsed Bush, Bush can be expected to join the Squad of progressive young House members who have been willing to break with House Speaker Nancy Pelosi (D-Calif.) to advance their agenda.

Clay is a member of the Congressional Progressive Caucus with a relatively liberal voting record. But he is also a major recipient of corporate PAC money, and critics insist that that cash has swayed him to take the side of business in key cases. For example, Clay opposed the Obama administrations fiduciary rule, which forced wealth managers to give advice that is in their clients financial interest.

The group Fight Corporate Monopolies invested nearly $100,000 in an ad blasting Clay for his opposition to the rule. And Justice Democrats, a left-wing group backing Bush, spent $150,000 on TV advertising promoting Bush.

Clay may have also been hampered by his reliance on family members to perform essential campaign functions. His campaign paid his sisters law firm $180,000 this cycle alone more than one-quarter of his total fundraising haul.

This is a common pattern for entrenched incumbents. They dont have tough elections, so they use campaign money to enrich family members, Irene Lin, a veteran Democratic campaign manager, told HuffPost. The donors should ask where their money is going.