Ishiba also now has a much-emboldened opposition eager to push through policies long stymied by the LDP. Support ratings for his Cabinet have fallen to about 30%.
Here is a look at what's happening in Japan's tumultuous politics, and what it might mean for Ishiba and his government as they prepare to navigate a second term of U.S. President-elect Donald Trump.
Why did the vote in parliament take place?
A parliamentary vote for a new leader is mandatory within 30 days of a general election. In the past that was mostly ignored as the head of the LDP usually enjoyed a majority in the Lower House, the more powerful of Japan’s two-chamber parliament.
This time, though, because Ishiba's LDP and its junior coalition partner lost its majority in the recent election, the runoff on Monday couldn't be avoided — the first in 30 years.
What's next for the prime minister?
Opposition's top leader, Yoshihiko Noda, has noted that nearly half of all lower house steering committees are now headed by the opposition. That’s a huge change from the pre-election domination of the LDP, which controlled all but three of the 27 committees.
“We are going to have a new landscape in Japanese politics,” Noda said.
Twelve of the committees in key areas, including budget, political reforms, national security and legal affairs, will be headed by Noda’s Constitutional Democratic Party of Japan and two other main opposition groups.
What's certain is that the era of LDP’s one-sided rule is over, for now, and the opposition has a chance to achieve policies long opposed by the ruling conservatives, including on issues like gender equality and diversity.
Noda said last Friday that a legal committee now headed by his party’s gender equality chief, Chinami Nishimura, is aiming to achieve a civil code revision to allow married couples the option of keeping separate surnames. That change has been stalled by LDP conservatives for 30 years, despite widespread support by the public and a United Nations panel on discrimination against women.
Who is the opposition kingmaker?
Yuichiro Tamaki is head of the conservative Democratic Party for the People, which quadrupled its seats to 28 in the election. The vote elevated his party from a fringe group to a major player.
He is now being cast as a potential key to Ishiba’s survival.
A Harvard-educated former Finance Ministry bureaucrat, the 55-year-old Tamaki has seen success by pushing for the raising of a basic tax-free income allowance and an increase of take-home wages. His messages on social media have appealed to younger voters, who have long been ignored by LDP policies catering to conservative elderly.
Ishiba apparently seems to find Tamaki’s 28-member DPP an attractive partner to secure a majority. The two parties, which have common ground in some areas — including support for greater nuclear energy use and a stronger military — have started policy talks.
Ishiba met with both Tamaki and Noda on Monday but Tamaki may be cautious about moving too close to a scandal-plagued LDP ahead of another election next year. Noda is struggling to form a unified opposition to force a change of government, which he says is his next goal.
What does this mean for Ishiba's government?
For Ishiba, the “hung parliament” requires him to win over opposition forces so he can push his policies. While considered unstable, it might also provide a chance for a more consensus-based policy making process, experts say.
“I’m taking the current situation positively as a chance to get our opposition voice heard more carefully,” Tamaki said.
Ishiba also faces challenges of restoring unity in his own party. A number of senior LDP lawmakers are waiting to overthrow Ishiba, though their priority is to resolidify their footholds, not infighting — and nobody is eager to do damage control at this difficult time anyway.
“The (Ishiba) administration is quite unstable. ... He will have to get opposition parties' cooperation every time he wants to get a bill approved, which could stall policies,” said University of Tokyo political science professor Yu Uchiyama.
And even if Ishiba survives politically in the coming months, there could be a call for his replacement ahead of next elections.
“Japan is likely to return to a period of short-lived government,” Uchiyama said.
How does this affect Japan's diplomacy, security and ties with Trump?
Ishiba congratulated Trump hours after his victory and in a brief telephone conversation, they agreed to closely work together to further elevate their alliance.
While experts say Trump understands the importance of U.S.-Japan relations, he may — as he did in his first administration — pressure Japan to pay more for the cost of 50,000 U.S. troops in Japan or to buy more expensive American weapons.
Trump's possible tariff proposals could also hurt Japanese exporters.
Ishiba on Saturday renewed his pledge to pursue an ongoing military buildup plan under a strategy that calls for a counter-strike capability with long-range cruise-missiles. He has long advocated a more equal Japan-U.S. security alliance but could face difficulty pursuing those plans.
”It will be a fantastic experiment to see if a national unity government can get Japan through until the next election,” said Michael Cucek, an expert in Japanese politics at Temple University in Japan.
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