The announcement today by Democratic Party leader Renho comes as her party is spearheading opposition questioning of Prime Minister Shinzo Abe and his Cabinet over the scandals.
Renho was her party's first female leader and was considered a possible future prime minister when 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 this month.
The party is seen as failing to appeal to voters looking for a viable alternative to Abe's conservative policies. Renho, a former TV personality known for her sharp tongue, uses one name.
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