“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.