Amending the Constitution requires the support of two-thirds of both houses of Parliament. Mr. Abe’s party and its allies had those numbers before Sunday’s elections, but the prime minister’s political woes earlier this year, along with public doubt about a constitutional change, raised the possibility that he would lose the supermajority in the lower house.
The victory on Sunday could also embolden Mr. Abe to run next year for a third term as leader of the Liberal Democrats, which could make him Japan’s longest-serving prime minister.
The results were a setback for Ms. Koike, who started her new party, Kibou no To, or Party of Hope, with great fanfare just hours before Mr. Abe called the early election last month. But after she decided not to run for office, voters lost interest.
Analysts said Mr. Abe’s victory did not represent an endorsement of his platform so much as a lack of strong alternatives.
“The story of this election, it would seem, is Abe didn’t so much win it as the opposition just was totally unprepared,” said Jeff Kingston, the director of Asian studies at Temple University in Tokyo.
Mr. Abe’s public approval ratings dipped below 30 percent over the summer as he was dogged by a series of scandals, and opinion polls taken during the campaign found that more voters disapproved of Mr. Abe’s hawkish strategy toward North Korea than approved of it.
“There is an Abe conundrum,” Professor Kingston said. “How does a guy who is basically unpopular with voters, whose policies are not particularly popular, who doesn’t get high marks for leadership, and yet he keeps winning in elections?”
Ms. Koike, after starting her own party, probably helped Mr. Abe by setting off a further split in the opposition. The leading opposition Democratic Party initially offered to free all of its candidates to run under the banner of Ms. Koike’s party. But after she said she would submit candidates to a litmus test and require them to sign a loyalty pledge, the left wing of the Democrats split off and formed yet another new party, the Constitutional Democratic Party, which gained some momentum during the campaign.
Analysts said that since Japan’s electoral system is based on awarding victories to candidates who get the most votes in a constituency, the proliferation of parties favored the incumbent Liberal Democrats, who have dominated Japanese politics for most of Japan’s postwar era.
“If the rival parties of the L.D.P. are divided, the L.D.P. wins,” said Koichi Nakano, a political-science professor at Sophia University in Tokyo. With two new parties forming just weeks before the election, Professor Nakano said, “basically people got confused.”
At a time when the economy has been slowly improving, North Korean missiles have been flying over Japan, and some here worry about the unpredictability of the United States under President Trump, voters chose stability.
“The L.D.P. has been serving such a long time and knows what to do,” said Natsuyo Kobayashi, 38, a caregiver at a facility for the disabled who cast a vote for the Liberal Democratic candidate in Sayama, a suburban town outside Tokyo. “And I think Japan should become a country that can protect itself with amending the Constitution. Missiles have been flying over, but I don’t think the U.S. will actually come to protect us.”