-- The Japanese prime minister, Junichiro Koizumi, put his job on the line Monday, dissolving the lower house of parliament and calling snap elections for Sept. 11 in an effort to win a new mandate for sweeping reform of the world's second-largest economy.

The move came after ruling party legislators rebelled against the prime minister's plan to privatize Japan's gargantuan postal service, effectively blocking the centerpiece of Koizumi's economic reform crusade. The decision sets up a public vote of confidence on Koizumi's 41/2-year administration as well as his efforts to restyle the Liberal Democratic Party, which has ruled Japan for most of era since World War II.

Koizumi, the Bush administration's closest ally in Asia and Japan's longest-ruling prime minister since the 1980s, said he would step down if his allies in the ruling coalition were defeated. A defeat would likely hand power to the opposition Democratic Party, which has opposed Koizumi's support of the U.S. military effort in Iraq and demanded a withdrawal of American Marines from the island of Okinawa.

Analysts say Koizumi, a self-described maverick, is engaging in the kind of daring politics for which he has become known. He is risking his political life in hopes that the public will back his reform-minded Liberal Democratic candidates, finally allowing him to crush party hard-liners who have strongly resisted his reforms from the start. The power struggle came to a head Monday after a large LDP faction in the upper house revolted against his postal service privatization plan.

Much more than stamp emporiums, Japan's web of post offices essentially form the world's largest bank, with millions of local depositors and assets totaling more than $3 trillion. For decades, LDP politicians have used it as a back door for costly construction deals and other pork-barrel projects that have helped them win votes in their home districts. But the system has served as a drag on the economy.

Koizumi has argued that the only way to get Japan surging again after a 14-year economic slump is to slash the nation's massive bureaucracy, particularly the postal service. On Monday, he appeared to be making good on his long-held promise to force change from within Japan's ruling party -- or be the one responsible for its downfall.

"Four years ago, I said I'd change the LDP," Koizumi told reporters. "And if it doesn't change, I'll destroy it."

In a nation where consensus politics has long been the rule, the wavy-haired, 63-year-old Koizumi swept into power in April 2001 on a vow to extinguish Japan's political dinosaurs. He became a national darling, as popular as a rock star, with looks to match. But his public support has ebbed ever since, with many seeing him as making only sloth-like progress on his promises. Analysts say Koizumi seems to be betting on opinion polls that put his personal popularity ratings above 40 percent -- significantly higher than either the LDP's or the Democratic Party's.

Koizumi's tough approach has bitterly divided the LDP. He exerted enormous pressure on party members to fall in line with his privatization legislation, which passed the lower house last month by a razor-thin margin. One party legislator verbally attacked by peers for his support of the measure committed suicide after casting his vote.

Some Koizumi critics cited his shogun style and his early threats to call snap elections as reasons for voting against the bill Monday. Koizumi on Monday fired Agriculture Minister Yoshinobu Shimamura, the most vocal member of cabinet opposed to new elections.

Koizumi has pledged to ban all LDP legislators who voted against his reform proposal from running on the party's ticket next month. While that could ensure that only those who support him get elected, those who do not are now openly discussing a break with the LDP that could bring down both the party and Koizumi.

"For four and a half years, he has kept up the contradiction between his own position as head of the party and the party's opposition to his reforms," said Takayoshi Miyagawa, president of the Center for Political Public Relations. "Now, that contradiction has finally exploded."

Koizumi is dissolving the lower house because Japanese prime ministers have no authority to disband the upper house, and, by his own calculation, the upper house will have to approve his bill if the public backs him.

But the last time the LDP entered an election this divisive was in 1993, when it briefly lost power. But in Japan's multi-party system, the Democratic Party -- which merged with Japan's Liberal Party in 2003 -- has emerged as a viable political alternative. Even if the Democrats dethrone the LDP, however, analysts say they will almost certainly need to forge a partnership with one of several smaller parties to form a coalition government.

The wild card remains the Japanese public. Polls show most here don't yet fully grasp Koizumi's complicated plan to privatize the postal service, and rural centers -- an LDP stronghold -- fear the plan could mean the closure of distant branches and a loss of vital services. That could hurt Koizumi. But few are counting him out.

Koizumi may also be able to woo back alienated conservatives. He hinted on Monday that he may soon stage another visit to Tokyo's Yasukuni Shrine honoring Japan's military dead, including convicted war criminals. Past Koizumi visits to the site have outraged China and South Korea, which were invaded by Japan -- but a visit now could win him back crucial support among Japan's right wing.

Special correspondent Sachiko Sakamaki contributed to this report.

Koizumi put job at risk in dissolving parliament and calling elections.Japan's web of post offices essentially form the world's largest bank, with millions of local depositors and assets totaling more than $3 trillion. It is the centerpiece of Prime Minister Junichiro Koizumi's privatization efforts.