TOKYO: Japan's embattled Prime Minister Taro Aso faces a crucial test in a Tokyo municipal election Sunday, seen as a bellwether for a coming national vote in which the ruling party risks losing power after governing for virtually all the past 50 years.

Aso's Liberal Democratic Party has lost four straight regional elections since April to the main opposition Democratic Party of Japan.

The 68-year-old prime minister has become so unpopular that some lawmakers within his own party are now calling for his removal before a general election, which must take place by October at the latest.

If the ruling party and its coalition partner, the New Komeito Party, finish poorly in Sunday's poll for the 127-seat Tokyo Metropolitan Assembly, pressure could further mount on Aso, whose approval ratings are at around 20 percent.

"Voters know Aso lacks leadership. Like previous local polls, the opposition party is likely to win the Tokyo election," said Yoshinobu Yamamoto, a political scientist at Aoyama Gakuin University in Tokyo.

Aso's party currently holds 48 seats in the Tokyo assembly, with the coalition partner Komeito having 22. The opposition party has 34.

Although Aso acknowledged the Tokyo poll would be "a tough battle," he rejected any link between Sunday's vote and the upcoming general election for the powerful lower house of parliament.

"The Tokyo election is a local poll. It is the judgment of various local issues by Tokyo residents. As I have repeatedly said, it is not directly linked to national politics," he said at a news conference Friday in the Italian city of L'Aquila where the G-8 summit was held.

"There is no change in terms of fulfilling my duties as the prime minister," said Aso, who took office in September 2008.

But opinion polls by Japanese newspapers show that the Democratic Party of Japan is well-placed to make major gains or even rise to power in the national election, with opposition leader Yukio Hatoyama likely to replace Aso.

That would mean the end of rule by the Liberal Democratic Party, which has governed Japan for all of the past 50 years except a brief period in 1993.

However, that would still require a sea change in voter support. The Liberal Democrats currently has 303 seats in the 480-seat lower house, and its partner Komeito has 31. The Democratic Party has just 112.

A nationwide survey by the Yomiuri newspaper, Japan's top-selling daily, showed Friday that 41 percent of respondents said they would vote for the opposition party in the national election, compared to just 24 percent for Aso's party.

Nearly 46 percent of respondents said 62-year-old Hatoyama is fit for the prime minister, compared with 21 percent favoring Aso. The Yomiuri survey of random voting-age people was conducted July 7-9. It received 1,087 responses.

The paper did not provide a margin of error, but based on the number of respondents the margin of error was plus or minus 4 percentage points.