Japan will hold key upper-house parliamentary elections on Sunday that are widely seen as a referendum on Prime Minister Junichiro Koizumi.

The once-popular Koizumi's approval ratings have declined after a series of controversial decisions, including his determination to keep Japanese troops in Iraq.

Leading analysts said Koizumi's ruling Liberal Democratic Party -- the conservative political force that has ruled Japan for most of the post-World War II era -- will be hard pressed to make gains or even hold on to its 50 seats up for election in the upper house of the parliament, the Diet.

Meanwhile, Japan's largest opposition force -- the Democratic Party of Japan -- is projected to win more than its 38 seats being contested. There are races for half of the 242 seats in the House of Councillors.

The results, analysts said, are likely to cement the Democratic Party of Japan's emergence as a powerful counterweight to the Liberal Democrats, marking an evolution here to a two-party system very similar to that of the United States. Though Japan's prime minister is typically chosen in lower-house elections, analysts said a crushing defeat for the Liberal Democrats could jeopardize Koizumi's job.

Voters are increasingly polarized over Koizumi, 62, according to analysts. His emphasis on personality -- from his trademark wavy gray mane to his direct, even daring political style -- has long separated Koizumi from the more stoic, traditional Japanese leaders of the past half-century. But while it has endeared him to some, Koizumi's flash, experts said, is beginning to backfire -- particularly as the prime minister has been seen as using his savvy to deflect scandal and controversy.

Koizumi, for instance, announced a surprise trip to Pyongyang for a summit with the North Korean leader, Kim Jong Il, in May just as a scandal was breaking over his inclusion in a group of politicians who failed to make some payments to the national pension system. The revelations emerged as the Liberal Democrats were pushing an unpopular pension reform bill through the Diet. Koizumi succeeded in bringing back five children of Japanese citizens kidnapped by North Korea in the 1970s and '80s for use in spy-training camps. But he could not shake the impression that the trip had been designed to divert attention from the pension scandal.

In the same vein, many Japanese are questioning the timing of a reunion on Friday in Jakarta between Charles Robert Jenkins -- a U.S. soldier who deserted to North Korea in 1965 -- and his wife, who was one of the kidnapped Japanese and who returned to Tokyo without her husband and daughters in October 2002.

"Anything that would draw out North Korea is positive, but this is so blatantly about politics that I feel uncomfortable with it," said Naoko Iinuma, 56, a housewife, discussing the Jenkins case as she left a high-end department store in Tokyo's flashy Shibuya district. Though she has voted for the Liberal Democrats in the past, she said she would switch to the Democratic Party on Sunday.

Koizumi has also been slipping in opinion polls -- his approval rating now stands between 37 and 40 percent, down from about 50 percent just two months ago -- partly because of his stance on Iraq.

Koizumi dispatched the Self-Defense Forces to Iraq this year in Japan's largest military-related deployment since World War II. Although the Japanese troops are on a noncombat mission there, involved largely in water distribution and supply transport, the mission has divided Japan, whose constitution renounces war.

Koizumi shocked many Japanese by pledging to President Bush at the Group of Eight summit in Sea Island, Ga., last month that the Self-Defense Forces would participate in the multinational rebuilding effort following the transition of power in Iraq. Koizumi made the promise before he briefed many of his party's leaders, sparking a new controversy.

"As a result of these elections, Koizumi will probably emerge as a weaker leader," said Akikazu Hashimoto, professor of politics at the National Graduate Institute for Policy Studies in Tokyo. "But he is saved by the fact that there is nobody popular enough to succeed him."

Special correspondent Akiko Yamamoto contributed to this report.

Junichiro Koizumi waves to supporters after making a campaign speech in Ikoma, near Osaka. His approval rating has fallen at least 10 points in two months.