Japan’s Prime Minister Naoto Kan marked one year in office on Wednesday, matching or outlasting his four predecessors, but mired in a deepening squabble over when he too will head for the exit.

Amid Japan’s political gridlock, and as it struggles to tackle recovery from the March 11 earthquake, tsunami and nuclear disasters, calls have been growing for a national unity government of both major parties.

Many see its as the only way for Japan to tackle key challenges — reviving the world’s third biggest economy, which was thrown back into recession by the disaster, and whittling down a massive public debt mountain.

Despite the daunting tasks ahead Japan’s Government has been distracted by its own political survival.

Kan last week defeated a no-confidence motion by pledging to step down once the quake recovery takes hold, but without giving a firm date, a move that placated rebels within his own party who had threatened to oust him.

Since then, Japan’s political class and media have been feverishly speculating over his possible departure date and replacement, dismaying the nation with their infighting and power plays at a time of crisis.

A Mainichi Shimbun editorial denounced the bickering with a headline that told politicians, “Don't become the laughing stock of the world”, and charged that the “power games show no concern for the victims” of the quake.

For Japan, a revolving door at the top has become standard since its best-known recent leader, Elvis-loving Junichiro Koizumi of the conservative Liberal Democratic Party (LDP), stepped down after a five-year term in 2006.

Kan’s centre-left Democratic Party of Japan (DPJ) took power in a landslide in 2009, ending the LDP’s almost unbroken rule of more than half a century with pledges for a new brand of people-centred politics.

Its first Premier, millionaire-politician Yukio Hatoyama, lasted less than nine months, and Kan has been limping along with approval ratings that dived below 20 per cent shortly before the quake.

“The disaster effectively extended his term as he had faced strong pressure to step down before March 11,” Tetsuro Kato, politics professor at Tokyo’s Waseda University, told AFP.

“But that effect did not last long. The disaster proved that Japan’s politics didn’t work in terms of crisis management.”

Many in his own party, including Hatoyama, want Kan to leave the scene this summer, once a quake reconstruction bill passes through Parliament, followed by financing bills that could be blocked by the opposition.

“Although the no-confidence motion was rejected, the possibility is high that Prime Minister Kan will resign as early as this month and no later than August,” said BNP Paribas Securities chief economist Ryutaro Kono.

However, even if Kan steps aside, Kono pointed to the threat of prolonged gridlock as the upper house is controlled by the LDP and its small partner the Buddhist-backed New Komeito.

“Despite a change in premiership, more than half of the seats in the upper house will remain controlled by the opposition,” he said in a note. “A new leader will have to seek the support of the LDP and New Komeito.”

The chairman of the powerful Japan Business Federation (Keidanren), Hiromasa Yonekura, is among prominent voices who have called for a grand coalition in which the DPJ and LDP would join hands for the first time.

“Speeding up reconstruction work in a visible fashion requires strong political leadership,” Yonekura said. “We would like the ruling and opposition parties to cooperate and jointly promote reconstruction.”

The idea of a unity government until 2013 elections has gained support in newspaper editorials and from heavyweights of both the LDP and DPJ, with the ruling party signalling a willingness to drop some policies opposed by the LDP.

Kato, of Waseda University, said: “A grand coalition is possible, but it should be limited to a certain period to handle certain issues, first and foremost reconstruction measures.”

Names being floated to succeed Kan include former foreign minister Seiji Maehara, DPJ secretary general Katsuya Okada, finance minister Yoshihiko Noda, chief Cabinet secretary Yukio Edano, and his deputy Yoshito Sengoku.

However, no clear frontrunner has emerged.

For now, Professor Kato predicted more instability: “Kan’s successor will face a tough time ahead. No matter who will succeed him, it is hard to see stable premiership under the current circumstances.”