TOKYO -- Japanese Prime Minister Taro Aso and his ruling party suffered fresh setbacks as the opposition won a closely-watched mayoral race and surveys showed his support rates had sunk again ahead of a election due within months.

Sunday's local poll bolsters expectations of a Democratic win in the general election and comes as renewed criticism of Aso's lack of leadership has sparked speculation that his party may seek to dump him ahead of the general election.

Kunio Hatoyama, a close Aso ally, resigned as internal affairs minister on Friday after a weeks-long feud about the management of Japan Post, which runs the post office.

He was the third minister to resign since Aso, 68, took office last September.

A survey by broadcaster TBS showed nearly three out of four voters did not back Aso's cabinet, up eight percentage points from last month, while about 80 percent disapproved of his handling of the feud with his cabinet minister.

Some 40 percent in the TBS poll preferred opposition Democratic Party leader Yukio Hatoyama over Aso as next prime minister and almost half said a change in government should be achieved in the next nationwide election, due by October.

A Democratic Party victory in the general election would end more than five decades of almost unbroken rule by the conservative LDP and bring to power a party which has pledged to pay more heed to the interests of consumers and workers than corporations.

Japanese media speculated that Aso's latest troubles could spark moves in the LDP to oust him, especially if the party fares poorly in a July 12 Tokyo metropolitan assembly election being billed as a bellwether for the national poll.

"If there are going to be moves to oust Aso before the general election, it would most probably be after the Tokyo election," said Koichi Nakano, a professor at Sophia University in Tokyo. "Of course, that is late, but it could be quite lethal for Aso because he's putting his weight behind it."

In the second local election victory for the Democrats since they replaced their scandal-tainted leader last month, candidate Toshihito Kumagai, 31, defeated a rival supported by Aso's Liberal Democratic Party (LDP) to become mayor of the city of Chiba near Tokyo.

Public opinion polls have shown the Democrats well ahead of the LDP in the run-up to a general election that must be held by October and that many expect to be held in August.

A June 12-14 survey by broadcaster NTV showed 38.9 percent of voters planned to vote for the Democrats in the general election versus 26.3 percent for the LDP.