Prime Minister Junichiro Koizumi's (search) triumph in parliamentary polls handed the leader a new mandate Monday to harness his revitalized ruling party and turn promises into action for a range of sweeping economic reforms.

His landslide victory Sunday boosted his Liberal Democratic Party's standing in the lawmaking lower house by nearly 20 percent and gave ruling lawmakers a two-thirds majority — along with a coalition partner — to override votes in a still-hostile upper house.

The LDP's final tally stood at 296 seats in the lower house, public broadcaster NHK reported, well above the 241 seats needed for a majority and the 249 seats it held when Koizumi dissolved the chamber last month. Optimism about the results sent Tokyo's benchmark Nikkei (search) stock index surging 1.8 percent in early trading.

Koizumi quickly came under pressure to use his new strength to deliver — not just on his cherished plans to privatize the nation's postal savings and insurance system but on issues ranging from pension reform to diplomatic relations.

"If his policies and the party's stature betray the people's expectations, there will someday be a backlash," the Asahi (search) newspaper said in a front page analysis. "As soon as possible, he has to say what he will do after postal reform, and show concrete programs."

Koizumi, a staunch ally of President Bush (search), is expected to stand by Tokyo's dispatch in support of the U.S.-led coalition in Iraq and is also a strong supporter of the continued presence of 50,000 U.S. military personnel in Japan. The opposition Democrats had pledged to withdraw troops from Iraq.

The LDP victory delayed any notion that Japan was entering an era of two-party politics following impressive recent gains by the opposition Democratic Party. The Democrats took a disheartening plunge on Sunday to 113 seats from 175. Party leader Katsuya Okada announced early Monday that he would step down as party head.

The Democrats plan to elect a new president Saturday, party officials said.

The LDP victory will test Koizumi's ability to transform the party's once-moribund, pork-barrel politics into a streamlined force for dynamic reform and small government.

Koizumi insisted again Monday that he will retire in September, but said he wants the next LDP president to carry his reform torch.

Koizumi plans to call a special session of Parliament as soon as Sept. 21 to again tackle postal privatization, Chief Cabinet Secretary Hiroyuki Hosoda said. The legislation was slapped down last month in the upper house with the help of rebels from within his own party. Koizumi plans to reshuffle his Cabinet after the special session ends, Kyodo reported.

This time, numbers are on his side. Combined with seats from ally New Komei Party, the ruling coalition now has more than 320 seats — a two-thirds majority to override votes in the upper house.

Official results were expected early Monday morning, but election officials said a minor counting error in one prefecture was delaying their release. Voter turnout surged more than 7 points from the last elections in 2003 to 67.5 percent, election officials said.

The public also is eager to see the government address the pension problem, deteriorating diplomatic relations with China and South Korea, and Japan's commitment to sending troops to Iraq.

South Korean President Roh Moo-hyun sent Koizumi a congratulatory message, calling the victory a reflection of the Japanese people's evaluation of his leadership and faith in reform.

In dry, diplomatic language, however, Roh added that he hopes "relations between the two countries will develop in a future-oriented and constructive manner."

The comment apparently reflected widespread concern in South Korea about Koizumi's visits to a wartime shrine that some say glorifies Tokyo's militaristic past and disputes over history that have soured bilateral ties.

The government in China, where massive and sometimes violent street protests broke out earlier this year over Japan's stance on its wartime aggression, did not immediately react to Koizumi's win.

Tokyo, Seoul and Beijing are among the United States' negotiating partners in efforts to disarm North Korea of its nuclear weapons.

Koizumi tried to deflect criticism that he was too focused on postal privatization at the expense of other reforms.

"I have been working on privatization along with other issues such as social security, the issues important to the people of Japan," Koizumi said Monday. "But as I have been saying, if you don't privatize the post, how can you move ahead with other reforms?"

Proponents of postal reform say privatizing its savings and insurance programs with $3 trillion in deposits — making it the world's biggest financial institution — will put that money into more efficient investments and produce a bigger boost for Japan's economy, which is the world's second-biggest but has stagnated for years.

The plans resonate with a public worried that bloated government bureaucracies are sapping economic growth while the aging of the population raises questions about how Japan will pay for future retirees. Postal savings have long been used by the LDP as a slush fund for public works projects.