TOKYO – Moves to oust unpopular Japanese Prime Minister Taro Aso will intensify if, as many expect, his ruling bloc fares badly in a key local election on Sunday that is considered a bellwether for a coming national poll.

The main opposition Democratic Party is ahead of Aso's long-ruling Liberal Democratic Party (LDP) in opinion polls ahead of the closely watched election for the Tokyo Metropolitan Assembly, mirroring its lead for the upcoming national election.

Depending on the degree of defeat on Sunday, there will be panic, Hiroshige Seko, an upper house LDP lawmaker and vocal Aso critic, told Reuters in an interview.

First, we will call for an early LDP leadership election and a change in the rules to allow that.

Aso's term as party chief expires in September and his critics want a vote to replace him moved to early August.

Deep doubts, however, persist over whether even that desperate measure would rescue the ruling bloc from defeat in a general election for parliament's lower house due by October.

Even then, it will be extremely difficult, Seko said. The mood is already set and it is hard for us to find a message to compete with the Democrats' call for a change in government.

A Democratic victory in the lower house election would end a half-century of nearly unbroken rule by the conservative LDP and raise the chances of resolving a deadlock in a divided parliament as Japan tries to recover from recession.

A survey of 1,087 voters by the Yomiuri newspaper published on Friday showed that 41 percent planned to vote for the Democrats in the national election compared to 24 percent for the LDP. That was in line with a survey by the Nikkei business daily.

POSSIBLE REPLACEMENTS

Public support for the 68-year-old Aso has eroded since he took office in September following a series of flip-flops over policies as Japan struggles with its worst recession in 60 years, and is now hovering around 20 percent.

In the latest sign of public displeasure, 44 percent of voters in the Yomiuri survey said Aso should quit versus 39 percent who want him to stay. Even 37 percent of his party's own backers want a new leader.

Possible candidates to replace Aso include Minister of Health, Labour and Welfare Yoichi Masuzoe, 60, a former academic and TV commentator seen as competent and hardworking, and former defense minister Yuriko Koike, 56. She ran against Aso in last year's LDP race in a bid to become Japan's first female premier.

But Japan has had three prime ministers since popular Junichiro Koizumi led the LDP to a huge election victory in 2005, and voters may not be impressed by another change at the top.

Last year, we chose Aso, said Agriculture Minister Shigeru Ishiba, who stood against Aso in the leadership race. How can we say after less than a year that this was a mistake? When the ship is leaking, we should all be working together to figure out how to fix the hole and bail out water.

Aso, who returns from a G8 summit on Saturday, could call an election for early August to fend off moves to ditch him, or party elders could decide to keep him in the job. Opposition party executives are set to meet on Monday to consider submitting a no-confidence motion to parliament in an effort to further undermine Aso's authority.

One wild card in forecasting the general election is a funds reporting scandal surrounding Democratic Party leader Hatoyama, who has admitted that some people listed as donors were dead.

The LDP can play a lot of dirty tricks when they are desperate, said Sophia University professor Koichi Nakano.

Either they find someone novel to change the label on the stale party, or do something negative to scare voters away from the Democrats. Those are the only possibilities for the LDP to cling to power.

(Editing by Nick Macfie)

Japan