Ms Yuriko Koike has said she will step down as leader of opposition party Kibo no To (Party of Hope) to concentrate on her duties as governor of Tokyo.
Kibo no To, which suffered a chastising defeat at last month's general election, will be led by four-term Lower House lawmaker Yuichiro Tamaki, 48.
Ms Koike, who founded the party early last month, said yesterday: "I'd like to leave national affairs in the hands of our party lawmakers and so will give up my position as party leader. But I will continue to support the party in an appropriate manner."
She will focus on her duties as governor of Tokyo - a sprawling metropolis larger than Singapore, and with a population of nearly 14 million - in the lead-up to the 2020 Olympic and Paralympic Games.
Ms Koike will be in Singapore for a three-day visit from today as part of the Lee Kuan Yew Exchange Fellowship programme, which invites outstanding individuals for high-level visits. Her resignation came just five days after Mr Tamaki, formerly of the Democratic Party (DP), was elected co-leader at a party caucus last week.
In the Oct 22 election, Kibo no To fielded 235 candidates and won just 50 seats, seven seats fewer than what it had in the Lower House before the vote.
It did not become the largest opposition force in the Lower House, ceding that honour to the newer Constitutional Democratic Party of Japan (CDP), which fielded 78 candidates and won 55 seats.
Kibo no To was formed with much fanfare. But the initial roar soon turned into a whimper, as public trust was quickly eroded by Ms Koike's perceived arrogance and her party's vague populist policy promises. A public survey by broadcaster NHK on Monday showed only 3.2 per cent of respondents supported Kibo no To - down 2.2 percentage points from the previous survey conducted a week before the election.
The ruling Liberal Democratic Party (LDP) had the support of 37.1 per cent of respondents, up 4.3 points, and the CDP was supported by 9.6 per cent, up three points.
Ms Koike has, since being elected governor in July last year, portrayed herself as a Joan of Arc taking on the grey elite of Japan's political establishment. She was once a member of the LDP, and is a former environment minister and the country's first female defence minister.
She formed Kibo no To as a right-wing opposition to the LDP, although both parties have similar hawkish ideologies, differing only on issues such as tax and nuclear policy. She declined to step down as Tokyo governor to run for a Lower House seat.
Mr Tamaki, who represents the No. 2 district in Kagawa prefecture, is aligned with Ms Koike's vision of a constitutional revision and a more active military.
"We are a tolerant, conservative, reformist party," the Nikkei quoted him as saying last week on his election as party leader.
He also rejected any possibility of an alliance with former DP members who are now with the CDP, saying this was counter-productive.
Dr Sota Kato, executive director for policy research at the Tokyo Foundation think-tank, told The Straits Times that Ms Koike will face an uphill battle to regain the trust of voters in Tokyo, where her party was roundly beaten.
"This was a tactical move," he said. "She recognises that the only way to regain her credibility and popularity is to achieve some eye-grabbing successes as governor. I expect her to keep a low profile for a while and focus on her job, while waiting for another chance."