He added: “Although it will be a difficult election, in order to overcome these national problems, I have to listen to the people’s voice.”
Mr. Abe is taking advantage of his rising public approval ratings, which have recovered from a nadir reached over the summer. After his Liberal Democratic Party lost to Ms. Koike’s party, Tomin First, in Tokyo elections in July, Mr. Abe appeared at risk of losing the chance to become Japan’s longest-serving prime minister.
“He thought politically this is the right time to call the election,” said Ichiro Fujisaki, a former Japanese ambassador to Washington, “because his party had some problems, but the other party’s problems are a lot bigger.”
Polls show that close to two-thirds of the public disapproves of Mr. Abe’s accelerated timetable, given that he is not legally required to call an election until December 2018. But over the weekend, a Kyodo News poll found that voters who planned to cast their ballot for Mr. Abe’s Liberal Democrats outnumbered those who would vote for the Democratic Party by more than three to one.
Yet with 42 percent of those surveyed still undecided, Ms. Koike’s wild card of a party is likely to capture some of these voters. The governor herself will not be running for a lower house seat, but she will use her popularity to campaign for her party’s candidates.
In announcing the new party, which will be called Kibou no To — Party of Hope — Ms. Koike said that she wanted to increase female participation in society and work toward an energy policy that eliminated nuclear power and reduced carbon emissions to zero. She said she wanted to debate the revision of the Constitution, but she questioned the wisdom of focusing exclusively on the pacifist clause that is at the center of Mr. Abe’s ambitions.
Ms. Koike called into question Mr. Abe’s timing in calling for an election. “I see a big question mark on calling an election in the midst of the North Korean situation being so critical,” she said during a news conference on Monday. “I wonder if it’s appropriate in terms of crisis management for the country.”
Koichi Nakano, a political scientist at Sophia University in Tokyo, said Mr. Abe was taking advantage of the North Korean crisis. “He is using the North Korean card to his advantage to call an unnecessary snap election just because it suits him,” Mr. Nakano said.
Analysts say Mr. Abe has been presenting himself as a force for stability at a time when residents have been awoken by text alerts notifying them of missile launches. He is also able to leverage the fact that he is one of the few world leaders to maintain a close relationship with President Trump and manage the often unpredictable American president.
“North Korea helped Abe raise his support rate, giving the impression to people that Abe is the only man who can deal with the threat from North Korea,” said Harumi Arima, an independent political analyst.
Editorials in several mainstream newspapers accused Mr. Abe of cynical timing and of trying to avoid answering tough questions about a pair of influence-peddling scandals in which he is accused of giving favors to friends in the education sector.
But some analysts said such criticism could backfire if opponents did not present credible campaign platforms. “Elections are the place where lawmakers should talk about their ideas about Japan’s future,” Mr. Arima said. “There is no use if they only complain or grumble.”
Still, Mr. Abe’s call for an early election is not without risk. The Liberal Democrats currently hold two-thirds of the seats in the lower house, so even if they win a majority, they could still lose seats. Such losses could jeopardize Mr. Abe’s desire to serve a third term.
“If they lose more than a couple of seats, then in some ways the door is going to slam shut on Abe,” said Tobias Harris, a Japan analyst at Teneo Intelligence, a political risk consultancy based in New York. “And even if he serves out the rest of his term, the criticism that he called an election to solidify his personal status — and in the process cost the party a few seats — will be a lot more intense, and there will be a major question about whether he deserves another term.”
It is not clear whether Ms. Koike’s party would hurt Mr. Abe’s political fortunes or whether it could become a coalition partner for the Liberal Democrats.
So far, four members of the Democratic Party and one from the Liberal Democrats have announced their intention to join the new party, along with a handful of independents.
Those who have said they will join the new party are in favor of revising the pacifist Constitution. If Mr. Abe aligned with the new party, “he could claim that there is more and broader support for his proposals,” said Takuji Okubo, managing director and chief economist at Japan Macro Advisors.
In a bow to the sensitivity of the constitutional issue, Mr. Abe did not mention it during his election announcement.
Mr. Okubo said that Mr. Abe would benefit from the recent stabilization of Japan’s economy. Mr. Abe has also vowed to use a consumption tax increase to pay for child care and free university and college tuition.
At a meeting with economic policy advisers on Monday, Mr. Abe said he would develop a 2 trillion yen, or $17.8 billion, plan to help improve preschool education and other social services.
The official campaign will begin Oct. 10, and voting will take place on Oct. 22.