Cori Bush, a racial justice activist who was once homeless, has ousted a 10-term political veteran in Missouri’s Democratic primary election.
She defeated Congressman William Lacy Clay 49% – 46%. Either Mr Clay or his father, a civil rights activist, has held the seat since the 1960s.
An ordained minister and former nurse, Ms Bush would be the first black woman to represent Missouri in Congress.
Her win is the latest upset against establishment Democrats by newcomers.
She had campaigned for Senator Bernie Sanders during his presidential run.
Praising her on Twitter, Mr Sanders said Ms Bush would “take on the corporate elite of this country when she gets to Congress”.
Ms Bush, 44, had lost to Mr Clay, 64, during the 2018 primary in Missouri.
Mr Clay is a long-time lawmaker who has represented the St Louis area for two decades. His father is a co-founder of the Congressional Black Caucus.
Mr Clay campaigned on his congressional record while highlighting Ms Bush’s lack of political experience.
Her primary win all but guarantees her a spot in Congress, representing the strongly Democratic district.
It follows a number of shake-ups in the 2020 election cycle. In June, House Foreign Affairs Chairman Eliot Engel lost his New York seat to middle-school principal Jamaal Bowman.
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The wins show the influence of the Democratic left wing ahead of November’s presidential election, where moderate Joe Biden will face off against Republican President Donald Trump.
Ms Bush spoke to supporters on Tuesday, saying many people expected her to lose.
“They counted us out,” she said, according to CBS News. “I’m just the protester, I’m just the activist with no name, no title and no real money. That’s all they said that I was. But St Louis showed up today.”
Ms Bush had to quit her job at a preschool when she fell ill while pregnant with her second child in 2001. She and her then-husband were evicted from their home, and – along with their baby and young son – were homeless for several months, living out of their car. The pair eventually divorced.
Since then, Ms Bush has earned a degree in nursing and became a pastor, before also becoming a racial justice activist. Ms Bush led protests in Ferguson in 2014, following the police killing of unarmed black 18-year-old Michael Brown.
During her 2020 campaign, she was backed by the Justice Democrats, who campaigned for her friend and fellow progressive Alexandria Ocasio-Cortez in 2018.
The Justice Democrats group praised her win as a “Black Lives Matter organiser” defeating a “corporate-backed political dynasty”.
Primary elections also took place in Michigan, Arizona, Kansas and Washington state on Tuesday.
Former Kansas secretary of state Kris Kobach lost the state’s Republican primary Senate election to more moderate Congressman Roger Marshall.
The outcome is a relief to Republicans, who feared a win for Mr Kobach, a controversial conservative, could flip the Senate seat.
Mr Kobach lost the 2018 gubernatorial race to Democratic candidate Laura Kelly despite backing from President Trump.
In Michigan’s primary, Democratic Congresswoman Rashida Tlaib is leading comfortably in her rematch with Detroit City Council President Brenda Jones, with 90% of the votes counted.
Ms Tlaib is a member of the group of progressive first-term congresswomen known as “the squad”, along with Ms Ocasio-Cortes.