CNN  — 

Cori Bush, a progressive activist and veteran of the racial justice protest movement, defeated 20-year incumbent Missouri Rep. William Lacy Clay in a Democratic primary on Tuesday, a stunning victory for the party’s insurgent left.

The US House seat, based in St. Louis, has been held by Clay and his father, former Rep. William Clay Sr., one of the founders of the Congressional Black Caucus, since 1969. Bush, who challenged Clay in 2018 and lost, was the first candidate launched by Justice Democrats, the progressive group dedicated to toppling moderate Democratic congressional incumbents.

Bush’s victory will send another round of shockwaves through the Democratic caucus on Capitol Hill, where leadership has fought to stave off a new generation of combative progressives, who support “Medicare for All,” the Green New Deal and other economic and racial justice policies dismissed for decades by the party establishment.

“We’ve been called radicals, terrorists, we’ve been dismissed as an impossible fringe movement – that’s what they called us,” Bush said in her victory speech. “But now, we are a multiracial, multiethnic, multigenerational, mass movement united in demanding change, in demanding accountability, in demanding that our police, our government, our country recognize that Black lives do, indeed, matter.”

After falling short in 2018, Bush, who became an activist leader in Ferguson back in 2014, raised more money in 2020 and benefited from increased credibility – and visibility – with voters who saw the electoral successes of Reps. Alexandria Ocasio-Cortez, Ilhan Omar, Rashida Tlaib and Ayanna Pressley. The primary took place amid a national anti-racist uprising, following the killing of George Floyd by a police officer in Minnesota on Memorial Day, and the coronavirus pandemic, which has exacted a disproportionately deadly toll on communities of color.

“It is historic,” Bush told supporters after the race was called, “that this year, of all years, we’re sending a Black, working class, single mother who’s been fighting for Black lives since Ferguson, all the way to the halls of Congress.”

Speaking to CNN’s John Berman on “New Day” Wednesday, Bush cited her own experience with badly suffering from Covid-19 earlier this year as part of her desire to push for a “Medicare for All” health care plan. She said St. Louis was “ill-prepared” for the virus.

“We didn’t have what we needed,” Bush said, “and predominantly Black areas, we were lacking.”

In her second attempt to unseat Clay, who won in 2018 with 57% of the primary vote, Bush was backed by Vermont Sen. Bernie Sanders, the Sunrise Movement – a youth-led group that champions the Green New Deal – and other leftist and progressive leaders, including Jamaal Bowman, who ousted Rep. Eliot Engel in New York during the state’s June primary. Ocasio-Cortez, who endorsed Bush two years ago, did not get involved in the 2020 contest.

“She is a true progressive who stands with working people and will take on the corporate elite of this country when she gets to Congress,” Sanders tweeted after the race was called. In the run-up to the primary, the Vermont senator had helped her campaign raise money and featured her on a Zoom call with his convention delegates.

In a tweet after midnight, Bush wrote, “Not me, US.” – the slogan of Sanders’ two presidential campaigns.

On Wednesday morning, Ocasio-Cortez congratulated Bush on Twitter.

“You’ve shown that the power of grassroots, mass-movement politics is stronger than gatekeepers & big money,” she wrote, “it’s about advancing the tide of justice whose time has come.”

Unlike Ocasio-Cortez and Bowman, Bush defeated a Black lawmaker with deep connections to the community he, and his father, represented for 50 years. Clay had accused Bush of being beholden to Justice Democrats, but he also sought common cause with progressives, most notably in co-sponsoring the Green New Deal resolution written by Ocasio-Cortez and Massachusetts Sen. Ed Markey.

Sunrise Movement co-founder Varshini Prakash was among the progressive leaders celebrating Bush’s upset win, which got a boost from the group’s activists in Missouri.

“Pundits and the establishment didn’t see this one coming, but the people of St. Louis did,” Prakash said in a statement early Wednesday morning. “She ran on Defund the Police, the Green New Deal, and Medicare for All and defeated a multi-generational political dynasty who got too close to corporate donors and too far from the needs of his district.”

Matriarch, a progressive group dedicated to electing women from working and low-income backgrounds to Congress and which counts Bush as one of its founding members, called her win a signal that a “bigger, bolder squad of working-class women” were poised to exert their growing political power – and numbers – on Capitol Hill.

“Cori represents the next generation of Democrats that the party desperately needs: a working-class, Black Lives Matter activist who understands the pain points of our suffering communities,” founder Nomiki Konst said in a statement. “Cori sacrificed it all to run for office and stand up for her community.”

This story has been updated with additional developments.

CNN’s Sruthi Palaniappan contributed to this report.