Sanae Takaichi, a hard-line conservative Japanese lawmaker, won a critical leadership vote on Saturday, putting her on track to become Japan’s first female prime minister, a milestone in a country where women are vastly underrepresented in politics.

Ms. Takaichi, 64, emerged as the winner of an election by the ruling Liberal Democratic Party after two rounds of voting by lawmakers and rank-and-file members.

She will succeed Shigeru Ishiba, Japan’s prime minister, who announced last month his intent to resign after less than a year in office, with his party being battered in the past two elections that saw it become a minority in both houses of the parliament, known as the Diet.

Ms. Takaichi beat out four other candidates, including Shinjiro Koizumi, the 44-year-old agricultural minister who was seen as the favorite heading into Saturday’s election. She was an ally of Shinzo Abe, the former prime minister assassinated in 2022.

Ms. Takaichi will face a series of challenges, including managing a tense and unpredictable relationship with the United States.

Japan, after finalizing a trade deal with the Trump administration in July, received a lower-than-threatened 15 percent across-the-board tariff on its exports in exchange for a commitment to inject $550 billion into the U.S. economy.

While the two countries have recently resolved several ambiguities in the trade agreement, a significant wrinkle remains: the details of how Japan will deploy that $550 billion in investment, loans, and loan guarantees.

Ms. Takaichi was the only candidate in the campaign who left open the possibility of renegotiating the deal, saying that if unfair practices emerge, “We must speak out firmly.”