Linda Edwards is the family authority on all matters of politics. Every election year, she watches the news, studies the candidates, attends campaign events and renders a verdict.

A year before the 2020 election, the 68-year-old retired pharmacist from Charlotte has her work cut out: 19 Democrats vying to be the Democratic presidential nominee. Yet with five months left before voting begins in the primary race, Edwards says she is ready to make an endorsement.

“Elizabeth Warren is the absolute greatest,” Edwards said of the Massachusetts senator after waiting for more than an hour to take a selfie with her at a recent campaign event in Rock Hill, South Carolina. “I always had her at the top of the list but she is the No 1 now. I totally support her.”

Since entering the race nine months ago, Warren has steadily gained ground with ambitious policy proposals, a decision to swear-off high-dollar fundraising events and her hours-long selfie lines. But if she is to win the nomination, it will likely be with the help of African American voters such as Edwards, part of an increasingly powerful and decisive constituency in the Democratic party.

A spate of recent polls show Warren edging past Joe Biden in the first two early-voting states of Iowa and New Hampshire, home to predominantly white electorates. But in South Carolina, where African American voters make up an estimated 60% of Democratic primary voters, Biden still enjoys a wide lead.

In South Carolina, known as the Palmetto State, which holds the “first in the south” primary on 29 February next year, Biden leads Warren by 21 points, according to a CNN poll released this week. Although they draw the same share of support from white primary voters in the state, 45% of black Democrats back Biden compared to just 4% who favor Warren.

“I don’t know how anyone can become the Democratic nominee – or the next president of the United States, for that matter – without strong, across-the-board support from African American voters,” said Antjuan Seawright, a Democratic strategist in South Carolina, who is not aligned with a candidate. “South Carolina is the first test of that support.”

The event at Clinton College in Rock Hill on Saturday highlighted the challenge for Warren as she works to introduce herself to African Americans in the state.

Despite the unbearable heat and humidity, nearly 1,400 attended her outdoor rally, and hundreds stayed afterward for selfies. Yet the crowd that blanketed the campus of this historically black college was overwhelmingly white.

Elizabeth Warren addresses the crowd at the event at Clinton College in Rock Hill, South Carolina. Photograph: Meg Kinnard/Associated Press

Biden drew a smaller, but more diverse, crowd when he visited the college earlier this year.

Biden’s dominance in the state rests on his deep ties to black political leaders and his service as Barack Obama’s vice-president, which have made him popular among older, more conservative black voters.

“We trust him,” said Steve Love, a local councilman in neighboring York, who endorsed Biden. “Obama is not got going to elect a vice-president who doesn’t have our back.”

Love met Warren before her rally and offered “point-blank” advice.

“If you want to make gains in our community, you are really going to have to come into our community and sit down and talk to us,” he told her.

Warren says she has plans to do just that.

“What I’m doing is showing up and trying to talk to people about why I’m in this fight, about what’s broken, about how to fix it and how we’re building a grassroots movement to get it done,” Warren told reporters after the rally. “It’s not just one policy. It’s everywhere.”

Woven into her raft of police proposals are specific prescriptions to address racial injustice. Her proposal to forgive most student loan debt and make college tuition-free attempts to reduce the racial wealth gap that disproportionately burdens black students. The proposal would also invest $50m in historically black colleges and universities (HBCUs), such as Clinton.

Her affordable housing policy specifically aims to redress decades of discriminatory housing practices and redlining in places such as the Mississippi Delta, where she went early in her campaign to highlight the initiative.

She was one of the first candidates to endorse congressional legislation that would create a commission to study reparations for the descendants of slaves. And at a recent forum on LGBTQ issues in Iowa last week, Warren began her remarks by reading the names of 18 black transgender women killed this year. “It is time for a president of the United States of America to say their names,” she said.

“Black folks have a very unique experience that requires policies that pinpoint that experience,” said Maurice Mitchell, the national director of the Working Families party, a progressive political organization that endorsed Warren.

“The candidates who aren’t afraid to talk about race and class at the same time, those are the candidates that are going to compel black people to not just show up at the polls,” he continued, “but to get involved, to volunteer, to engage and to build a movement with them.”

A woman listens to Democratic presidential candidate Elizabeth Warren at an event in New Hampshire. Photograph: Cheryl Senter/Associated Press

Black voters, and black women in particular, are the most loyal Democratic voting bloc. In 2016, African Americans comprised nearly a quarter, 24%, of Democratic primary voters – a share that is expected to rise in 2020.

There are signs Warren’s efforts are paying off, especially among African American women.

A Quinnipiac poll showed that her support among black voters nationally climbed over the summer from 4% in July to 19% in September as Biden’s support slipped from 53% in July to 40% in September.

At several presidential forums focused on voters of color and in private meetings with activists and black leaders this year, Warren has left her audiences impressed, said Aimee Allison, founder of She the People, political advocacy group focused on women of color that hosted an event with 2020 candidates in Houston earlier this year.

“She is campaigning directly to women of color,” she said. “And at the same time, she is attracting white progressives. That has the potential to be a potent coalition.”

Cliff Albright, cofounder of Black Voters Matter, said fresh scrutiny of Biden’s record and his recent remarks on race – from comments about working with segregationists to a discordant reply to a debate question about reparations – are starting to chip away at his support, especially among younger black voters.

“The more that black people hear from Elizabeth Warren, the more they are intrigued by her,” Albright said. “The opposite happens with Joe Biden.”

But public opinion surveys and interviews with voters suggest Biden’s appeal may be more durable than many expect.

Melissa Rouse, 46 and Tracey Easter, 44, cousins from Charlotte who sat in folding chairs under the shade of a tree as they waited for Warren to speak, said they have not yet settled on a candidate, but Warren was at the top of their list.

That wasn’t the case for many of their older relatives, who they said are firmly committed to Biden.

“My mom is 76 and she loves, loves, loves Joe Biden,” Rouse said. “They feel like they know him.”

Both said they thought Biden would be the strongest candidate against Trump. Nevertheless, they came to be persuaded by Warren.

“When people have an opportunity to be in her presence and hear her message, they always leave impressed,” said Wendy Brawley, a South Carolina state representative who has endorsed Warren. “Now I’m starting to hear, ‘This is a person who I not only like and support, but who can actually win.’”

Before leaving Rock Hill, Warren made a final stop for dinner at Gourmet Soul Kitchen. As cooks rushed to prepare an order of fried shrimp and hush puppies, Warren worked the room, introducing herself to staff and diners, all of whom were black.

Deborah Cousar, a 60-year-old retired nursing assistant who had rushed to the restaurant with her grandchildren upon hearing of the senator’s visit, beamed as Warren told her 11-year-old granddaughter that she was running for president because “that’s what girls do”.

Though their encounter was brief, it left an impression on Cousar. While she intends to hear out the other candidates, especially as the primary race for South Carolina intensifies, Cousar said the “vibrant lady” from Massachusetts will be hard to beat.

“She’s on fire,” Cousar said. “If she just keeps on doing what she’s doing, I think she’s going to persuade them pretty good.”