SEOUL, South Korea — Preliminary tallies in Japan’s snap general election on Sunday indicated that new Prime Minister Shigeru Ishiba’s conservative coalition would lose its majority in the Diet’s House of Representatives.

According to preliminary vote counts early Monday by national broadcaster NHK, with 68 seats still undeclared, Mr. Ishiba’s scandal-plagued Liberal Democratic Party had won 162 seats and its coalition junior partner Komeito had won 20, totaling 182 and securing the largest plurality of votes.

The 465-seat House requires 233 seats to be won for a ruling majority. Falling short of a majority does not mean a change of government, but the results would make it difficult for Mr. Ishiba to get his party’s policies through parliament, and he may need to find a third coalition partner, The Associated Press reported.

Mr. Ishiba, who became prime minister on Oct. 1, immediately called for a general election to legitimize his new leadership and distance himself from a lingering slush fund scandal that engulfed his Liberal Democratic Party.

LDP lawmakers had been found to have pocketed yen from political fundraising events last year, when the party was headed by then-Prime Minister Fumio Kishida, who decided not to run in the intraparty leadership race.

Sunday’s snap election was a bold move by Mr. Ishiba, who could have held off calling for a vote until October 2025.

Voters — many of them disillusioned by LDP corruption and battered by inflation — opted to punish the ruling party.

Before Sunday’s snap election, the LPD had held 247 seats and Komeito 32, for a total of 297. The main opposition party, the leftist Constitutional Democratic Party, had held only 98.

According to NHK’s preliminary tallies, the Constitutional Democratic Party had won 134 seats, while the Japan Innovation Party and the Democratic for the People Party captured a combined total of 55 seats.

Former LDP members who succeeded as independent candidates in Sunday’s elections could be pivotal in a future coalition.

“The LDP may reinstate some candidates who have secured seats as independents,” NHK reported. “The party did not endorse them in light of the fundraising scandal.”

Japan, a key U.S. ally, occupies strategic terrain in a region where China, North Korea and Russia converge.

The island democracy provides facilities for U.S. forces in South Korea, while its southern islands cover Taiwan’s northeastern flank. It hosts the largest contingent of U.S. troops stationed outside the United States, and is strengthening defense ties with regional democracies such as Australia, the Philippines and South Korea.

The conservative LDP has overseen a major defense buildup since finessing a “reinterpretation” of Japan’s pacifist constitution in 2015-2016.

Mr. Ishiba seeks a more equitable Status of Forces Agreement — the legal basis for U.S. troops in Japan — but is widely seen as florid on defense. He is strongly pro-Taiwan and has even floated the possibility of creating a NATO-style multilateral alliance in East Asia.

“I think Ishiba may have miscalculated the timing to hold [the election],” said Haruko Satoh, an academic who teaches Japan’s relation with the region at the Osaka School of International Public Policy. “He may have had in mind shaking off [late premier Shinzo] Abe-faction lawmakers while the scandal was still a hot issue with voters.”

That stance alienated some conservative voters.

“As Ishiba focused on financial scandals for the election, many LDP supporters gave up on the LDP and voted for opposition parties such as the CDP,” said Akira Yasui, a salesman from Nagoya who voted for the fringe Conservative Party of Japan.

“They are the true conservative party, cherishing our tradition,” said Mr. Yasui, who cited its stance on a male-lineage emperor, a promise to revise Japan’s peace constitution, and its tough postures on LGBTQs and immigration, for his support.

If Mr. Ishiba loses conservative traction and is required to wheel and deal with minority parties, his freedom of action could be constrained.

One issue is Mr. Kishida’s pledge to almost double Japan’s defense spending by 2027. Questions about where that funding would come from remain unanswered.