The leader of Japan’s main Opposition party said on Thursday that she was stepping down, citing her failure to regain voter support despite a series of scandals buffeting the ruling party.

The announcement by Democratic Party leader Renho comes as her party is spearheading Opposition questioning of Prime Minister Shinzo Abe and his Cabinet over the scandals.

Ms. Renho became her party’s first female leader and was considered a possible future Prime Minister, when the Democrats were in power from 2009 to 2012. Since then they have lost significant ground in parliamentary and local elections, including Tokyo city assembly elections earlier this month.

Ms. Renho, a former TV personality who uses one name and is known for her sharp tongue, said she decided to step down after only 10 months to help reunite and strengthen the party.

There was no immediate announcement of her successor.

Meanwhile, the popularity of Mr. Abe’s administration, which has plunged over recent scandals, suffered another blow as Defence Minister Tomomi Inada, a protégé of Mr. Abe, is likely to step down on Friday over the alleged cover-up of military documents from UN peacekeeping operations.

The Defence Ministry is accused of hiding parts of a daily log of activities by Japanese UN peacekeepers in South Sudan that referred to dangers faced by the troops, a sensitive topic.