When eight Republicans voted to take down Kevin McCarthy as speaker, seven of the votes weren’t a surprise. But one—from South Carolina Rep. Nancy Mace—sent a gasp through the House chamber.

       Unlike the other Republicans who joined all Democrats to vote against McCarthy on Tuesday, Mace is no hard-line conservative. She justified her vote by saying McCarthy hadn’t brought up legislation he had promised he would, including a measure that would increase access to birth control, and echoed dissidents’ complaints that he broke his pledges on spending bills.

       “Promises that I was made were not kept. I have been working very hard on very specific issues for a very long time. When you shake a hand and make a promise, you ought to keep it," Mace said in an interview Wednesday.

       Mace, 45 years old, has been seen as a rising star within the party. A charismatic lawmaker who regularly gives media interviews and is aggressive on social media, she sometimes elicits eye rolls from fellow Republicans who feel she is too eager for attention. At the same time, they acknowledge she is a talented member who held on to a tough seat.

       Mace has been vocal about how the party could win back the moderate Republicans and independents turned off by former President Donald Trump. She has wide appeal in her district, with a focus on fiscal conservatism and social moderation, pushing back against her party on abortion and marijuana policy.

       The lawmaker, who has indicated an interest in running for Senate or governor one day, has been advised that she needs to shore up her support from conservatives to have a chance, according to people familiar with the conversations. South Carolina is much more conservative statewide than the Charleston district she represents.

       Mace, the first female graduate of the Citadel military academy, was a star recruit in 2020 when she flipped the Charleston area district from a Democrat, helped by her compelling personal story. Before being accepted to the Citadel, she had dropped out of high school and earned her diploma while working at a Waffle House.

       She won her first congressional race by campaigning on promises of fiscal responsibility, but with a willingness to break with her party when needed. She said that is what she did Tuesday when she voted to oust McCarthy.

       McCarthy’s allies see Mace’s vote to oust him as the most stinging of the eight, given she was a favorite of leadership and had been recruited by McCarthy, according to a person familiar with the matter.

       The former speaker, for his part, accused Mace of lying. “I bite my lip. I let people say things that are not true. But it’s not right," he said after his ouster Tuesday.

       He said he had spoken with Mace’s chief of staff ahead of the removal vote, and that the aide assured McCarthy he had kept his word as it related to Mace’s requests.

       Asked to respond to that account, Mace said: “It was sad to see him fall apart the way he did last night." She said she and the aide in question were drinking together at a nearby bar while McCarthy gave his press conference.

       Mace, the only Republican woman to oppose McCarthy, may have angered her colleagues—210 Republicans voted to keep McCarthy, after all—but the next speaker will still need her vote. Her opposition to McCarthy underscored that while his sharpest critics were among the hard-right members of his party, other members had soured on him as well. Now, she could be an influential vote.

       She said she had heard from Rep. Jim Jordan (R., Ohio), the chairman of the Judiciary Committee, and she was working on setting up a meeting with Rep. Steve Scalise (R., La.), the majority leader. Both have announced a run for the post.

       Might the next speaker hold a grudge over her vote against the party’s former leader? “They can’t afford to, the majority is so slim," she said.

       Her decision Tuesday punctuated an extraordinary falling out between two former allies. McCarthy had supported Mace through her competitive races, including a 2022 primary where Trump tried to take her out. Mace returned the favor by mostly siding with the GOP on contentious votes.

       Voters in Mace’s wealthy coastal district tend to be fiscally conservative but more socially liberal, and Mace has made changing the way Republicans talk about abortion a priority. While she said she opposes the procedure, she believes Republicans need to support exceptions, including for rape, incest and the life of the mother. She has also been adamant that the party offer alternatives so there are fewer unwanted pregnancies, including access to birth control.

       The approach seems to be working. In 2022, Mace dispatched a Trump-endorsed rival in the GOP primary, and cruised to victory in the general election. While Republicans nationally took back the House majority, they won fewer seats than hoped. In the Senate, they lost a seat, a result attributed in part to GOP moves to restrict abortion after the Supreme Court overturned Roe v. Wade.

       “I’m fiscally conservative, socially sensible. I understand firsthand what was important in last year’s elections," she said.

       Mace has shifted alliances before. She worked for Trump’s 2016 campaign and ran her 2020 race focused on his policies. But shortly after she arrived in Washington, a pro-Trump mob overran the Capitol in an effort to stop certification of President Biden’s election. Mace, who had voted to certify the result, said Trump’s actions on Jan. 6, 2021, “wiped out" his legacy. She called for the party to be rebuilt.

       Three years later, Mace is considering endorsing Trump for president, and her name is being thrown around as a potential running mate. She has said she would be interested in the job if it were offered, people familiar with her thinking have said.

       Write to Eliza Collins at eliza.collins@wsj.com

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       Nancy Mace Explains Why She Nixed Kevin McCarthy as House Speaker