Momentum grew in Japan's ruling party on Friday to back 71-year-old lawmaker Yasuo Fukuda, known for his pro-Asian diplomacy, as successor to Prime Minister Shinzo Abe after his shock resignation.

'I think this is an emergency situation. I don't know how it will develop ... but I want to seek your support,' Fukuda, a former chief cabinet secretary, told colleagues amid reports that other party factions would also back him in the race for Liberal Democratic Party president, and hence prime minister.

The party poll will be held on Sept. 23 and the victor is assured of the premiership by virtue of the ruling camp's grip on parliament's lower house, which picks the prime minister.

Finance Minister Fukushiro Nukaga, 63, was the first to declare he would run but on Friday he decided to pull out and back Fukuda instead.

That put the focus on a battle between Fukuda and LDP Secretary-General Taro Aso, 66, a close Abe ally who shares many of the outgoing leader's hawkish views on security policies.

Abe's year in power was marred by scandals involving cabinet members and a humiliating election defeat in July that cost his ruling coalition its majority in parliament's upper house.

But his shock decision to step down stirred worries of delay in decisions on vital policies such as tax and fiscal reform.

'The main difference between Mr Aso and Mr Fukuda is likely to be in foreign policy,' said a report by Lehman Brothers. 'Mr Fukuda is known as a foreign policy dove (i.e. pro-China and Korea) while Mr Aso is seen to be on the other side.'

Whoever succeeds Abe will face potential deadlock in a divided parliament. 'This next (LDP) leader will be a sort of caretaker until the next general election,' said Hidenori Suezawa, chief government bond strategist at Daiwa SMBC.

'He won't put forth drastic policies, and they wouldn't pass anyway.'

Aso, a former foreign minister who shares much of Abe's conservative agenda to boost Japan's global security profile and revive traditional values, had been seen as frontrunner to succeed Abe when he stepped down. As the tide appeared to shift in Fukuda's favour, some Aso supporters questioned whether a decision based on factional dynamics would go down well with the Japanese public.

'Is it really OK to decide based on faction numbers?' said LDP lawmaker Kunio Hatoyama, speaking to reporters.

Known as a fan of 'manga' comic books and for his ability to work a crowd, Aso had been thought best placed among possible candidates to win popular support. But his closeness to Abe and his record for gaffes have raised doubts about his suitability for the post.

Despite media reports that Fukuda was in the lead, some cautioned that the outcome was still uncertain. - Reuters