Democratic Party lawmaker Shiori Yamao, a prominent adversary of Prime Minister Shinzo Abe in Diet debates, tendered her resignation from the party Thursday evening amid allegations of an extramarital affair, dealing a heavy blow to the nascent leadership of the struggling opposition.

“I’m deeply sorry my misleading behavior caused trouble to many people,” a somber Yamao told a hastily arranged news conference in Tokyo.

“I’ve decided to leave the party to avoid bringing any confusion to the upcoming extraordinary Diet session.”

The scandal concerning prosecutor-turned-lawmaker Yamao couldn’t come at a worse time, as Seiji Maehara, elected the DP president on Sept. 1, tries to hold together a party bedeviled by an exodus of lawmakers. The party also braces for key triple by-elections — largely considered a barometer of the next Lower House election — in October.

On Thursday, weekly tabloid Shukan Bunshun carried an article alleging that 43-year-old mother Yamao had been seeing Rintaro Kuramochi, a 34-year-old lawyer who is also married, meeting in secret about four times a week.

The magazine alleged that Yamao and Kuramochi stayed together at an upscale Tokyo hotel through the morning of Sept. 3 — just after Maehara became the new party leader.

Facing reporters, Yamao denied she was in a relationship with Kuramochi, insisting that she stayed at the hotel by herself. Once she read out a prepared statement, Yamao cut short the news conference, without so much as a single response to the barrage of questions from reporters as she hurried off into a waiting car.

On Thursday morning, Maehara had said that he will speak directly with Yamao to gain a fuller understanding of the situation.

The allegation leveled at two-term lawmaker Yamao, considered a young hopeful of the DP, is apparently what prompted Maehara to rethink his initial intention to select her as DP secretary-general earlier this week. He reportedly retracted Yamao’s appointment at the last minute, instead assigning senior lawmaker Atsushi Oshima to the No. 2 post.

Maehara’s eleventh-hour fumble was embarrassing enough, but Yamao’s alleged adultery, which was covered extensively by TV variety programs throughout Thursday, could further take the shine off the DP’s attempt at a fresh start.

“I feel sorry from the bottom of my heart that my actions have put a damper on vows of solidarity and revival we made” when Maehara was elected last week, Yamao said.

When asked about a possible adverse effect Yamao’s departure could have on the DP’s standing in the October by-elections, Oshima said it “cannot be denied” that the party will face an uphill battle due to the scandal.

A graduate of the prestigious University of Tokyo, Yamao rocketed to fame last year when she grilled Abe in the Diet over a shortage of child care facilities, citing an anonymous online posting by a Tokyo mother who wrote: “Didn’t get a slot in day care. Drop dead, Japan!”

The phrase went viral and eventually made a list of the year’s top 10 buzzwords, with Yamao receiving the award on the mother’s behalf.Yamao’s position on key issues — such as the day care crunch and sex crime laws — has built her image as a champion of women.

Her burgeoning stardom also resulted in her landing a position as DP policy chief in March last year under the previous leadership of Katsuya Okada.

She suffered a brief setback, however, when suspicions arose over exorbitant fuel expenses claimed by her office. Tabloid Shukan Shincho said the expenses would amount to her traveling 210,000 km, or five times around the Earth. Yamao blamed the suspicious figures on her former secretary.

In the past, lawmakers have been felled by exposed adultery. Liberal Democratic Party lawmaker Kensuke Miyazaki quit his seat in February last year after admitting to an extramarital affair with a bikini model. The revelations came only months after he declared his intention to apply for paternity leave, denting his reputation as a caring husband. In April this year, Toshinao Nakagawa, another LDP lawmaker, also resigned from the party amid allegations of infidelity.

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