TOKYO — Japan's struggling prime minister called Monday for national elections next month following a crushing defeat for his party in a Tokyo municipal election seen as a barometer of voter sentiment.

With the opposition surging in popularity — on the back of economic malaise and several gaffes by the ruling party — it appeared increasingly likely it could take power after 50 years of almost exclusive rule by Prime Minister Taro Aso's Liberal Democratic Party.

Aso told leaders of his party he would likely dissolve the powerful lower house of the legislature next week, with a general election to be held on Aug. 30, according to a party spokesman.

The emboldened opposition, led by the rising Democratic Party of Japan, added to his headaches Monday by submitting to parliament a no-confidence motion, said party spokesman Toshiaki Oikawa. The motion was expected to have little real effect other than to embarrass the administration and delay discussions on pending bills.

The political maneuvers came after elections results released early in the morning showed Aso's Liberal Democrats and their ruling coalition partner lost their majority in the Tokyo Metropolitan Assembly, the local parliament for Japan's capital and most populated prefecture. The opposition made major gains.

The Liberal Democrats began faltering a couple of years ago and have seen a revolving door of prime ministers — Aso's two predecessors each lasted less than a year. Since taking office in September, Aso, too, has failed to revitalize the party, which many say has simply been in power too long to effectively energize voters seeking change in the face of the financial downturn.

His popularity, which remains near 20 percent, has also been hurt by a series of scandals, including accusations one of his ministers turned up at a news conference drunk.

The Tokyo vote does not directly affect national elections, but it was closely watched as a bellwether for the parliamentary vote. The prime minister campaigned in support of party candidates, but his party lost to the opposition for the fifth straight regional election since April.

"The results of the Tokyo Metropolitan Assembly election were very bad," said Tsuneo Watanabe, a senior fellow at the Tokyo Foundation think tank. "We will have a new prime minister in the next month."

He said Japan's new leader could come from either a victory by the opposition or be chosen by the ruling party to replace Aso and lead its bid to stay in power. In the Japanese political system, the head of the party that has control of the national parliament generally becomes prime minister.