The defeat of longtime Missouri Rep. William Lacy Clay by Black Lives Matter activist Cori Bush took some Democrats by surprise, confounding their ideas of who was and wasn’t vulnerable to a left-wing primary challenge.

It did not surprise the movements and organizations that pushed Bush over the finish line. The win by the 44-year-old nurse and pastor, one of several liberal victories in Tuesday’s primaries, came after her own dogged campaigning was boosted by relatively new liberal PACs and by robust fundraising that caught the incumbent — a member of a dynasty that represented St. Louis for 51 years — by surprise.

“They counted us out,” Bush told supporters Tuesday night at her St. Louis campaign office. “They called me just a protester. I’m just an activist, with no name, no title and no money. That’s all they said I was. But St. Louis showed up.”

Bush is the fifth left-wing Democrat to oust an incumbent member of Congress from her party since the start of Donald Trump’s presidency. In April 2017, she became the first recruit announced by Justice Democrats, an organization built by veterans of Sen. Bernie Sanders’s 2016 presidential campaign. Each candidate embraced the key planks of Sanders’s platform — Medicare-for-all, criminal justice and immigration reform, as well as opposition to corporate PAC money. Each was also a serious underdog.

Bush’s win was a breakthrough for a movement that had fallen short in other races against members of the Congressional Black Caucus. In April, after Justice Democrat Morgan Harper lost a race against Columbus, Ohio, Rep. Joyce Beatty, the CBC touted its success and said it had stepped up its political organizing to protect Black incumbents from challengers.

“Cori Bush took down a political dynasty on her second try,” tweeted Kayla Reed, the executive director of Action St. Louis, a group that grew out of the Ferguson protests. “She ended a 50-year stronghold. Do you understand what that means? The entire Black political establishment came after her, Reed said. “And she put them in the ground.”

On paper, Bush was the sort of candidate whom party committees politely ignore. After becoming deeply involved in the 2014 protests that followed the police killing of Michael Brown in the St. Louis suburb of Ferguson, Bush made a quixotic 2016 run for U.S. Senate, raising less than $7,000 and winning just 13 percent of the vote in that year’s Democratic primary.

Justice Democrats gave her some campaign infrastructure and access to a national donor network. She was followed on the trail by the makers of “Knock Down the House,” a documentary about female, liberal, working-class congressional challengers. When now-Rep. Alexandria Ocasio-Cortez won her June 2018 primary, she cited Bush in her victory speech and soon traveled to St. Louis to campaign for her. (Ocasio-Cortez did not weigh in on the race this year.)

“I’m a single parent, so I understand taking care of children by yourself,” Bush said in a 2017 video introducing her House campaign. “I know what it’s like to live paycheck to paycheck.”

Clay, whose father had held the seat for decades before him, did not engage as fully in the race as some other challenged incumbents. According to Federal Election Commission reports, he had spent less than $550,000 on the race by mid-July, holding on to hundreds of thousands of dollars despite the lack of real Republican competition in November. Bush tripled her fundraising and spending from the last cycle, beating Clay to the airwaves with an ad that asked why she was in the streets fighting for racial justice, and he wasn’t.

Bush was also boosted by Justice Democrats’ own PAC and by Fight Corporate Monopolies, a new PAC formed after the end of the Sanders campaign this year, whose TV ad accused Clay of siding with the financial industry and against Barack Obama to hurt low-wage workers. Sanders himself endorsed Bush, who traveled the country as a surrogate for his 2020 campaign, and used his own social media platform to host a rally for her. Sanders also tapped his donor list, raising $107,000 for Bush.

“It’s really a story of starting all the way from the bottom and coming out on top,” said Waleed Shahid, a spokesman for Justice Democrats. “First, Black Lives Matter is just way more popular than when it started. Secondly, we’ve been working to create an ecosystem of support for all of our candidates, including paid media to get the message out about the race.”

By early July, the race began to look winnable for Bush. A poll conducted by Data for Progress found Clay ahead of Bush, but with only 39 percent support and with more than a third of voters undecided. The same poll found Obama’s favorability among the district’s voters at 90 percent, which informed an 11th-hour attack ad against Clay’s financial reform vote. Weeks later, the same poll found that the race was tied.

Clay never effectively rallied to save his job. In a Wednesday morning statement, the congressman’s father said that the family’s “years of elected public service [has] ended” and blamed “outside money from sources associated with Bernie Sanders.” But Clay never pulled outside groups into the district to combat the late ad-spending for Bush.

That wasn’t the case in other primaries this cycle, including those that the left has won. In New York, Justice Democrats-backed Jamaal Bowman was buffeted by ads from a PAC funded by allies of Rep. Eliot L. Engel; Bowman won that primary. In Texas, Rep. Henry Cuellar survived a close call with a Justice Democrat after the Chamber of Commerce swooped in to help. In Minnesota, which votes next week, Rep. Ilhan Omar has been outspent by Antone Melton-Meaux, an attorney who’s backed by the same PAC as Engel was and has raised millions of dollars off the congresswoman’s notoriety and criticism of Israel.

A defeat for Omar, who is backed by the state’s Democratic Farmer-Labor Party, would cut against the trend of liberal wins this year. Criminal justice reform advocates won county prosecutor races across the map on Tuesday, including in St. Louis, where Kim Gardner won a second term.

In Detroit, Rep. Rashida Tlaib declared victory over challenger Brenda Jones after she led by a 2-to-1 margin with 90 of precincts reporting, doubling her vote total from a crowded 2018 primary that convinced some Democrats that she was vulnerable. Jones, who had nearly defeated Tlaib that year, raised little money and only rarely campaigned in person after surviving covid-19. Bush, who also contracted the disease, quickly returned to the trail after recovering.

In both Detroit and St. Louis, turnout was also markedly up from 2018, despite pandemic conditions that had suppressed traditional campaigning. That was a setback for a theory some Democratic moderates had about the left — that it might win in low-turnout primaries but would run out of votes when a larger electorate showed up.