A record nine candidates are in the running after the long-powerful factions of the Liberal Democratic Party (LDP) disbanded earlier this year over a funding scandal.
Because the conservative LDP holds a parliamentary majority, the winner is certain to become prime minister, and will likely call a snap election to shore up their mandate.
Polls indicate a toss-up between ex-defence minister Shigeru Ishiba, hawkish Sanae Takaichi — a rare prominent woman in Japanese politics — and surfing political blue-blood Shinjiro Koizumi, who would be Japan’s youngest premier.
“This is the most unpredictable that an LDP election has been in many years,” Jeffrey J. Hall, a lecturer at Kanda University of International Studies, told AFP.
Whoever wins must face down regional security threats, from an increasingly assertive China and its deepening defence ties with Russia to North Korea’s banned missile tests.
At home, the leader will be tasked with breathing life into the economy, as the central bank moves away from decades of monetary easing that has slashed the value of the yen.
On the streets of Tokyo, 72-year-old retiree Yasumi Fujino told AFP she was “concerned about China” and hoped “the next person will focus on diplomacy”.
But “I do not agree with Takaichi… she’s like the number one follower of Prime Minister Abe”, she said, referring to Shinzo Abe, the nationalistic Japanese leader assassinated in 2022.
Fair and square’ Ishiba
LDP presidents are in office for three years and can serve up to three straight terms. Unpopular Prime Minister Fumio Kishida is not running for re-election.
With the factions dissolved, it’s unlikely any one candidate will win enough votes — split between lawmakers and rank-and-file party members — to win outright.
That makes a run-off between two top candidates the most likely scenario, with the winner announced on Friday afternoon.
Ishiba, 67, has come close to the top job before, including in 2012 when he lost to Abe.
The military model-maker with an affinity for 1970s pop idols says his experience tackling tough social issues, such as agriculture reforms, makes him qualified for the job.
As scandals fuel public discontent with the LDP, “the tide is in favour of Ishiba and his ‘fair and square’ attitude,” said Yu Uchiyama, a University of Tokyo politics professor.
Economic Security Minister Takaichi, 63, is opposed to changing the law to allow separate surnames for married couples, and regularly visits Tokyo’s Yasukuni war shrine — a flashpoint in relations with South Korea and China.
“Japan is completely looked down on by China,” she told Fuji Television when asked about the first confirmed incursion by a Chinese military aircraft into Japan’s airspace, which happened in August.
Naofumi Fujimura, a professor at Kobe University’s Graduate School of Law, said that while Takaichi currently needs the support of the LDP’s right-wingers, she could prove “more centrist, or more pragmatic” as prime minister.
Critics of former environment minister Koizumi, a keen surfer whose father was prime minister in the 2000s, say he is too inexperienced to lead the country.
Even so, the 43-year-old “best personifies the idea of rejuvenation and change for the LDP” among the frontrunners, Uchiyama said.
Other candidates include reformist Taro Kono, 61, chief cabinet secretary Yoshimasa Hayashi and Takayuki Kobayashi — the only other candidate under 50, seen as a wild card by some pundits.
Whoever emerges victorious on Friday will be formally elected by parliament on October 1.
The LDP has governed Japan almost uninterrupted for decades, with the main opposition parties rarely seen as viable alternatives.
During his term, Kishida has taken steps to double Japan’s defence spending, opening the door for military exports as the LDP seeks to revise the pacifist post-war constitution.
He welcomed Ukrainian President Volodymyr Zelensky to a G7 summit in Hiroshima and has strengthened Japan’s often testy ties with its neighbour South Korea.
But his rule was also tarnished by scandals, voter anger over rising prices and sliding poll ratings.