PHOENIX— Erika Kirk sat beside her husband in December at Turning Point USA’s annual conference as the couple explained how they reconciled building a nationwide conservative political powerhouse with their commitment to putting family first.

“I would not have been able to do any of it without Erika,” Charlie Kirk said. “I was going into battle, going into campuses, traveling the country. People that want us dead and they smear you and slander you and Erika is holding down the fort at home.”

That nightmare scenario came true last week when Charlie Kirk was shot and killed while speaking on a college campus, shattering the Kirk family and millions of Americans who have grieved alongside them.

“You have no idea the fire that you have ignited within this wife,” the widowed mother of two children, aged 1 and 3, said after her husband’s death. “I’ll make Turning Point USA the biggest thing that this nation has ever seen.”

Friends said Charlie Kirk, who was 31 years old when he was killed, had long envisioned a leading role for his wife at the organization if he died—a plan the couple hoped never to have to implement.

“They weren’t his plans. They were their plans,” said Arizona State Sen. Jake Hoffman, who has worked with Turning Point USA for years. “While he was the head of the organization, at some level she has had a hand at work in this plan for the organization since they’ve been married.”

Erika Kirk is taking on a much larger formal role at the organization than she had before, but friends say she acted as her husband’s most trusted adviser, in addition to making public appearances at his side. The couple had emphasized that women should give priority to raising children above career.

Friends say that she will be able to fashion a role at Turning Point while juggling parenthood. “She is more than prepared to take whatever steps she probably needs to take, or she’s willing to take, on behalf of Charlie,” said Brittany Kennada, a longtime friend who lives in Arizona.

Erika Kirk helped her husband build Turning Point USA into one of the most powerful conservative organizations in the country. Photo: Caitlin O’Hara for WSJ

A Turning Point representative declined a request for an interview with Erika Kirk.

Erika Kirk (nee Frantzve) was raised in Arizona by a single mother, who she recalls taught her the importance of prayer. She was a college basketball player and crowned Miss Arizona USA 2012.

She lived during her 20s in New York City, where she appeared briefly on the Bravo reality show “Summer House,” which follows New Yorkers who decamp to the Hamptons. “It’s tough to be a believer in the city,” she said on the show.

She met Charlie Kirk in 2018 at a New York burger restaurant for what was supposed to be an interview for a job at Turning Point. “Forget this job interview. I want to date you,” he would later recount saying. They married a few years later.

“There was a massive transformation in Charlie’s life after he met Erika,” said Alex Bruesewitz, a friend of the couple and adviser to Trump. “Charlie really got his priorities straight.”

In public, Erika Kirk and her husband emphasized her role as wife and mother, but she juggled that with a career of her own. She ran a Christian clothing line and an organization that encourages people to read the Bible every day. She also hosted a weekly podcast with episodes ranging from a few minutes to nearly an hour, discussing the Bible with a backdrop of ambient music.

Together the couple built Turning Point—which Charlie Kirk founded in 2012—into one of the most powerful conservative organizations in the country, with a presence on 3,500 college and high school campuses and a substantial media arm and voter-outreach program. Its influence has only grown since his death. Turning Point has received 62,000 inquiries from people looking to get involved or start new chapters, a spokeswoman said.

As a face of the conservative movement, Charlie Kirk was a charismatic speaker who could parry with college students across the political spectrum. He often played the role of combative provocateur, taking on hot-button issues such as race, gender, abortion and same-sex marriage.

“Young women who voted for Kamala Harris , they want careerism, consumerism and loneliness,” he said on Fox News, just a few days before his death.

Before her husband’s killing, Erika Kirk typically confined her public remarks to speaking about her faith and extolling her role as wife and mother. It was rare for her to discuss politics outside of this framework.

In May, she introduced her husband at an event for Rep. Andy Biggs, a GOP candidate for Arizona governor, by describing herself as a longtime Arizonan and a mom who supported a candidate aligned with her family’s values. “This is not my normal orbit, but Arizona is very special to me,” she said.

While Erika Kirk has a softer public image than her husband’s, she identifies as deeply conservative and is expected by those close to her to continue to lead the organization in that direction.

Charlie Kirk said on his podcast earlier this year that his wife was to his right politically. “By far, not even close. I am a moderate compared to Erika,” he said.

Even some progressives said Erika Kirk will be an effective leader and that they worried she could further expand the movement’s reach.

“Erika Kirk is going to turn up the volume on the right’s efforts to win young women in a big way—no one on our side is ready for this,” Amanda Litman, president of an organization that trains progressives to run for office, posted on X.

Leading this movement, even with momentum behind her, will likely pose challenges for Erika Kirk as she balances her role as a mother.

Kristan Hawkins, a national antiabortion leader who knew both the Kirks, said they were partners in expanding Turning Point and traveled together, including to a summit her group recently held. Still, she said the couple was adamant in their belief that children should be cared for by a parent. “They did very much place an importance, like I do, that the parent needs to be in the home with the kids,” she said.

The Kirks used their considerable platform to urge Americans to marry young and have large families and spoke frequently of holding traditional roles in marriage.

“People have asked, like, ‘How do you do it when Charlie’s traveling?’” Erika Kirk said at the AmericaFest conference last year. “It’s because we understand our biblical roles in our marriage. He is the head of the household and I am not a servant. I am not a slave to the master. I am his helpmate. I am the guardian of the home, that is my domain.”

Write to Eliza Collins at eliza.collins@wsj.com , Tali Arbel at tali.arbel@wsj.com and Alyssa Lukpat at alyssa.lukpat@wsj.com