By Roland Buerk
BBC News, Tokyo
Ozawa's popularity rating plummeted in the wake of the funding scandal
Ichiro Ozawa's long drawn-out departure as leader of Japan's main opposition may have handed Prime Minister Taro Aso something few thought possible at the start of the year - a real chance of winning the next election.
Mr Ozawa, head of the Democratic Party of Japan (DPJ), was ahead in the polls but in March his personal secretary was arrested and indicted for accepting illegal donations from a construction company.
He insisted he had done nothing wrong and hung on for months, before finally bowing to growing public distaste on Monday.
"He should have gone a long time ago," said Jeff Kingston of Temple University in Tokyo.
"All he did was amplify the damage to his own party, and turned it from almost a sure-fire victory to one where they're going to do not that well."
The scandal has been so damaging because it undermines everything for which the DPJ has stood - and happened just before elections which must be called by September at the latest.
The party promised a new politics, championing consumers and workers over corporate interests, breaking the stranglehold of the bureaucracy on policy, and pursuing a more assertive diplomacy towards Japan's main security ally, the United States.
A DPJ election victory would be a watershed in Japanese politics, ending the rule of Taro Aso's Liberal Democratic Party (LDP), which has lasted for more than half a century apart from a single break of less than a year.
Japan has been hit hard by the global economic downturn
But the allegation over money has been a powerful reminder of Mr Ozawa's own background. He was once an LDP strongman and, before he defected to the opposition, was mentored by some of the most corrupt leaders in Japan's modern history.
The DPJ held its nose while Mr Ozawa, once described by a former speaker of the Diet (parliament) as looking like "a toad which has just licked something terribly bitter", used his formidable skills as a political fixer to widen the party's support.
But an opinion poll this week showed 71% of respondents wanted him to go, more even than would like to see the back of the prime minister.
Disaffection with politics is widespread in Japan. Mr Aso is the fourth prime minister to take office since the last elections for the lower house in 2005.
Leaders are often seen as out of touch and incapable of ruling the country.
The fact that politics is unusually dynastic in Japan does not help. Mr Aso is the grandson of a prime minister, and his predecessors Shinzo Abe and Yasuo Fukuda were the grandson and son of prime ministers respectively.
In fact nearly 40% of LDP members in the lower house come from districts handed down through families, something that is far from unknown in the DPJ too.
Young scions of political dynasties inherit name recognition and powerful local election machinery, which makes it harder for newcomers to break into politics.
And many Japanese say the dearth of new blood shows in the quality of the country's leaders.
The main contenders to succeed Mr Ozawa are all political heavyweights
The disenchantment with Japan's leaders matters because the world's second biggest economy is in something of a political stalemate, just as it endures a sharp recession.
The opposition already controls the upper house of the Diet, meaning it can stall important legislation such as the 15 trillion yen ($155bn; £105bn) stimulus spending plan currently being debated.
Optimists in the DPJ say the party could now forge ahead to try to take the lower house too, without the headwind of Mr Ozawa's personal unpopularity.
Key will be who emerges as the new party president.
Many of the names being put forward are of people who have been tried as leader before: Naoto Kan, Yukio Hatoyama, Katsuya Okada and Seiji Maehara.
Whoever takes over could struggle to keep together the disparate interests that make up the DPJ party.
"It's essentially an amalgam of LDP retreads on the right, ranging all the way to the left-wing remnants of the socialist party, the union movement," said Prof Kingston. "So it's really hard for them to hammer out a cohesive platform.
"What was working for them was [that] the LDP was shooting itself in the foot over a range of issues. What is working for the LDP now is the economic crisis and the security crisis with North Korea. We're the safe pair of hands, they say. Do you trust the B-team?"
The Democratic Party of Japan may yet have cause to rue the loss of the deal-making and electoral-strategizing skills of Ichiro Ozawa.