ST. LOUIS, Mo. — Cori Bush was running the Alexandria Ocasio-Cortez playbook.
A progressive political outsider, Ms. Bush had promised “big change” for a St. Louis congressional district that has had the same representative for nearly two decades. She had championed Medicare for all, a $15 minimum wage and tuition-free public college. She had vowed to speak for those who felt unheard.
“We should have a voice in our community, and we should be important!” she told a handful of black business owners at a coffee shop here last week. “That’s something that I want to take to D.C.”
Now, just days before Tuesday’s primary against a powerful and long-tenured incumbent, Ms. Bush’s campaign has become a test case for how effectively the insurgent left can convert its energy into upheaval at the ballot box — and whether Ms. Ocasio-Cortez’s success in New York City can be replicated in other regions.
In Missouri, the conventional wisdom suggests this: The revolution may not have arrived in St. Louis just yet.
Ms. Bush, 42, a community activist, ordained pastor and nurse, is a decided underdog in her race against William Lacy Clay, who is seeking his 10th term in Congress. Strategists and officials here say Mr. Clay — whose father, Bill Clay, helped found the Congressional Black Caucus — is simply too respected, and too entrenched in the political fabric of this heavily Democratic district that is roughly 50 percent black. And they say a Midwestern state like Missouri is too steeped in tradition to adopt this new brand of progressivism that looks a lot like socialism.
“Trends get to the heartland late,” said Jay Nixon, the former Missouri governor and a Democrat, “and I think this is a trend that’s going to go over us.”
Ms. Bush, who made her name in the district as a prominent activist in the aftermath of the 2014 police shooting of Michael Brown in Ferguson, brushed off the idea that Mr. Clay would never lose.
“The theory that people say, ‘Well, that’s such a machine and we have to be loyal to who was there before’? I heard it a lot at first. I don’t hear it as much anymore,” she said in an interview between campaign events last week. “I’m hearing more people saying that we want change.”
Ms. Bush’s campaign, while conveying the passion of the far left, also underscores the growing divisions in the Democratic Party between those who believe the need for new thinking outweighs deference to party stalwarts, and those who defend the value of experience and tradition.
Some party leaders have cautioned that the insurgent left is practicing a form of protest politics that is not suited to actual governing.
“Being a progressive is one thing,” Representative James E. Clyburn of South Carolina, a member of the black caucus and the third-ranking Democrat, said in an interview. “Being practical with your progressiveness is another.”
The practicalities of winning races has factored into other primaries involving progressive black candidates. In Massachusetts, for instance, the political arm of the Congressional Black Caucus backed the incumbent, Michael E. Capuano, over Ayanna Pressley, a Boston City Council member who would be the state’s first nonwhite female House member. And Mr. Clyburn said he planned to campaign in Florida for Stephanie Murphy, a blue-dog Democrat facing a challenge from Chardo Richardson, an Air Force veteran who is backed by the progressive group Brand New Congress.
“I really don’t believe this thing is as generational as people seem to be wanting to make it,” Mr. Clyburn said. “It’s about your vision.”
Because of Mr. Clay’s stature in his Missouri district, Ms. Bush mostly avoids direct attacks on him — more often referring to him as “the incumbent” or “the other person.”
“It’s not about him,” she said after a late lunch at a White Castle. She walked gingerly, the result of a back busted by a bad car accident in April.
But she is unapologetic about challenging another African-American candidate, and a respected member of the Congressional Black Caucus at that.
“I didn’t expect the Democratic Party or anybody to support me,” she said. “I knew that this would just be totally the people.”
Mr. Clay, 62, who was first elected to the House in 2000, has dispatched challengers before and insists he will do so again in Tuesday’s primary. In 2016, in the wake of the protests in Ferguson, when the district was perhaps most agitating for change, Mr. Clay beat his main primary opponent with more than 60 percent of the vote.
“I smoked her,” Mr. Clay said in an interview after a ribbon-cutting ceremony for a new light rail station. “This is what I do. I’m used to this.”
He has won all of his nine general elections for the House with at least 70 percent of the vote.
He acknowledged, however, that while he considers himself a progressive, candidates like Ms. Bush and Ms. Ocasio-Cortez have changed the word’s definition, pushing it to the left. “I don’t know what the new litmus test is,” he said, “and I’m really not sure if I can pass that according to some.”
Born in St. Louis, Ms. Bush has campaigned on a compelling back story that she says makes her an ideal leader for her district. She said she is advocating for a $15 minimum wage because she knows what it is like to make $5.35 an hour. She wants to do more to support the homeless because she once lived out of her car, an old Ford Explorer. In the mornings, she said, she would go to a McDonald’s to wash up and mix formula for her infant daughter.
But she earned a nursing degree. And she became a pastor.
Ms. Bush is viewed as a political newcomer, but this is not her first time running for office. After the Ferguson shooting, she ran for the United States Senate in 2016. “It wasn’t a symbolic run — I was running to win,” she said.
She lost her primary by more than 50 points.
But Ms. Bush said she learned a lot from her defeat, including the importance of meeting people face-to-face.
Her run also attracted the attention of Brand New Congress, the progressive group that also backs Ms. Ocasio-Cortez; last year, Ms. Bush was the first 2018 candidate whom Brand New Congress endorsed.
Buoyed by Ms. Ocasio-Cortez’s victory and a visit she made to campaign with Ms. Bush last month, Ms. Bush has picked up some momentum.
She got a shout-out recently on “The Late Show with Stephen Colbert.” On Wednesday, she secured an endorsement from the progressive group Democracy for America. Nina Turner, the head of the Bernie Sanders-aligned advocacy group Our Revolution, campaigned with Ms. Bush this weekend.
Even those who back Ms. Bush expect many voters, and older black voters in particular, to support Mr. Clay.That has prompted Ms. Bush at times to tailor her message to a younger audience. When she filmed a radio ad last week, she had the producer underlay it with Cardi B’s “I Like It,” in part because she and her advisers thought it would help her appeal to young voters.
She was worried, however, about the optics of using a song with a Latin sound, because she said she had been accused of not caring enough about the black community.
“It’s almost like I should be solely focused on just issues that affect black people directly,” she said later. “We have to all work together.”
Despite the long odds Ms. Bush faces, there may be no better race to assess the actual power of an anti-establishment progressive message than here, where Mr. Clay, and his father before him, have held power for the last half-century.
Ms. Bush, for her part, remains undaunted, especially after seeing Ms. Ocasio-Cortez win.
“Before it was kind of like, ‘It is an uphill battle but you might be able to do it,’” she said. “Now, it’s like, ‘O.K., we think you’re going to do it.’”