The high-stakes race to define Kamala Harris

Which story voters believe will be crucial to the outcome of the November election between her and former President Donald Trump, her GOP rival. Though Harris has been in political life for 20 years—including four years in the Senate and nearly four as the vice president—she is still a relatively new figure on the national stage.

While her popularity has surged since her whirlwind ascent to the top of the Democratic ticket, a recent CBS News poll showed that over a third of voters said they didn’t know what she stands for. And Harris has only just begun laying out an agenda to guide what she will do if she wins.

“​​With Harris, it’s an opportunity. I think a lot of people would acknowledge there’s a lot they don’t know about her," said Karen Finney, who worked on Hillary Clinton’s 2016 campaign. “It’s not about just learning facts; it’s getting a feel for who she is."

Republicans have focused much of their criticism on Harris’s 2019 campaign and her record in California. “Kamala Harris is a radical San Francisco liberal who has spent her career advocating for weak-on-crime policies that have allowed criminals back on the streets and made our communities less safe," said Trump spokeswoman Karoline Leavitt. The GOP has also sought to tie her to Biden’s policies on the economy and inflation and decisions such as the chaotic withdrawal of troops from Afghanistan, which have been unpopular with voters.

Building up to Harris’s remarks Thursday night at the Democratic convention, her campaign has been using friends, colleagues and video testimonials to fill in the blanks some voters might have regarding her upbringing and her record. Aides also want to pull back the curtain on her personal life, on which Harris has often been reticent, showcasing her as a wife, daughter and stepmother.

Harris’s team wants to build on her momentum and dispel any concerns related to her unsuccessful 2019 Democratic presidential campaign or her struggles early on as vice president over such issues as immigration.

Her campaign has so far avoided detailing many policy specifics, protecting her from giving fodder to Republicans for attacks as she seeks to unite the party at its convention. Instead, the campaign has opted to release a broad vision to make housing more affordable while lowering the cost of groceries. Aides said they want her agenda to provide a contrast with Trump without reading like a think-tank white paper.

“I would expect in her remarks tomorrow, in the days and weeks that follow, that she’ll continue to flesh out the message," said Pennsylvania Gov. Josh Shapiro at an event hosted by Bloomberg on the sidelines of the convention. “I know she understands how to do that. The things that she’s laid out have been really popular."

The GOP vice-presidential nominee, Sen. JD Vance of Ohio, has criticized Harris for not taking more questions from reporters or providing more details on her policy plans. He also tried to latch her to Biden.

“She’s been a linchpin of some of the policies that’s driven up the cost of groceries," Vance told reporters Wednesday. “We just have to remind people: She is actually the vice president right now, as much as she’s trying to run from her own record."

Trump has repeatedly criticized her intelligence. “Our country needs a very smart person, and I don’t think she’s a very smart person," Trump said in an interview with CBS this week. He declared at a recent rally that he was a “better-looking person than Kamala."

Harris’s allies have expressed concern throughout her vice presidency that voters didn’t know who she was, giving Republicans, who saw her as a future political threat, the chance to define her. Her team is now trying to contrast her background with Trump’s, stressing a modest upbringing with a single mother, who didn’t buy a home until Harris was a teenager, compared with Trump, whose father was a real-estate developer.

“The neighborhood we grew up in was a very hardworking, middle-class, tightknit community," said Harris’s sister, Maya Harris, in a video aired Monday night. On Tuesday, Harris’s husband, Doug Emhoff, described a warm, family-focused person, who his children know as “Momala," and who officiated at his son’s wedding.

“Kamala put so much time into those remarks," Emhoff said of his wife’s role in her stepson’s nuptials, “and she bound them in a book that matched her dark red dress and then turned that into a gift for the happy couple."

Chauncey McLean, president of Future Forward, a super PAC backing Harris, said that its research has shown that voters are willing to give Democrats some “grace or openness" to learn more about Harris.

“Republicans were hoping that they were going to be able to paint her as more of the same," McLean said at a forum on the sidelines of the convention. McLean added that instead they have heard voters they have surveyed say, ‘Wait a second, she was vice president. She wasn’t president. And we want to see what she’s about, what’s her vision,’" he said.

Harris, the first woman to serve as vice president, who in the past has stressed the need to chart a path for more people like her, has largely left the history-making appeals to others.

“It’s pretty obvious, you don’t need to say it," said Sen. Kirsten Gillibrand (D., N.Y.). “And in fact, she’s showing it."

The choice contrasts with Clinton, who leaned into the historic nature of her campaign.

Democrats also have embraced Harris’s experience as a prosecutor, arguing that she has stood up for families and taken on big banks, calling her a lifelong protector.

During her unsuccessful 2019 presidential campaign, her aides were reluctant to lean into her early career as much as she tried to appeal to Democratic primary voters wary of her law-enforcement background.

Now, in a general election, Democrats are hoping her record in law enforcement draws a contrast with Trump, who was convicted in May on 34 felony counts, while also showing voters how she might lead the country.

“A lot of people are tired of Donald Trump. A lot of people don’t like him or JD Vance," said Bakari Sellers, a former Harris aide. “And so the question that they have is that they’re really unsure if Kamala Harris can lead the country. I think she has to articulate what a Harris presidency looks like."

Write to Catherine Lucey at catherine.lucey@wsj.com and Tarini Parti at tarini.parti@wsj.com