Ichiro Ozawa is seen as one of Japan's political veterans
Until his close aide was arrested on suspicion of accepting illegal donations, opposition leader Ichiro Ozawa was widely seen as the most likely candidate for Japan's next prime minister.
Just a few months ago he was riding a wave of popularity, capitalising on deep public discontent with the ruling Liberal Democratic Party (LDP), which was being blamed for the country's worst recession in decades.
But since the funding scandal broke in March, Mr Ozawa has been dogged by calls for his resignation.
He has finally acquiesced, saying he did not want to be a burden on the opposition Democratic Party of Japan (DPJ) ahead of the elections, which must be held before 10 September.
Mr Ozawa's father was an MP, and he also holds former prime ministers as some of his mentors.
He began his political career in the LDP, which has governed Japan almost uninterrupted since 1955.
In 1993 he chose to leave the LDP, upsetting the parliamentary balance and briefly forcing it from power, and set about trying to advocate modernisation and more assertive diplomacy towards the United States.
He joined the DPJ when it formed in 1998, representing both former socialists and others - like Mr Ozawa - disgruntled with the LDP.
He led the party to a landslide victory in upper house elections in 2007, and was widely expected to do so for the lower house elections later this year.
But the funding scandal, involving a charity which is said to have received illegal donations, has proved his undoing.
One of Mr Ozawa's senior aides and two company executives were arrested for political funding law violations.
Although Mr Ozawa himself has been neither questioned nor prosecuted in the case, for some senior party officials whether or not he was directly implicated was not the point.
In wake of the arrests, a contrite and tearful Mr Ozawa insisted he would stay on as party leader, dismissing the allegations as politically motivated.
His decision to quit was a significant U-turn, but perhaps not unexpected.
For a party trying to present voters with a clear - and clean - alternative to the current government, Mr Ozawa had to be above suspicion, some party officials have said.