Japan's largest newspaper, Yomiuri, puts support for the Prime Minister's Liberal Democratic Party at 50 per cent, compared with just 20 per cent for the main opposition, the Democratic Party of Japan (DPJ).
Most other polls also put the LDP well ahead, but slipping since it topped 53 per cent in mid-August.
The Prime Minister has promised to quit if his coalition fails to get "one seat less" than a majority.
Mr Koizumi called a snap general election for 11 September and expelled 37 members of his own party last month after they voted against his plan to privatise the mammoth Japan Post.
He has since repeatedly called the contest a battle for Japan's future, spinning himself as a radical reformer willing to risk the LDP's half-century grip on power to face down the party's old guard.
As a third-generation LDP politician, Mr Koizumi is an unlikely candidate finally to torpedo "Japan Inc" - the trinity of bureaucratic, political and business interests that have run the country for 50 years.
But if he succeeds, he will emerge as one of Japan's strongest political leaders since 1955, and clear the way to sell the post office, which boasts assets of more than $3 trillion (£1.64 trillion) and employs 300,000 people.
With so much at stake, the election has been predictably bitter. The former LDP policy chief Shizuka Kamei, one of the expelled rebels, called the Prime Minister a "fascist" and a "tyrant" on Wednesday. "He is endangering democracy in this country," he said.
Mr Kamei and other opponents accuse the Prime Minister of trying to import US-style capitalism and destroy Japan's social system. "Mr Koizumi's market reforms go against the fundamental DNA of our country, in which everyone helps each other and every person is precious," said Mr Kamei.
The opposition parties have tried to shift the focus of debate away from postal privatisation, where Mr Koizumi is on solid ground, toward the Prime Minister's disastrous foreign policies, which have sunk relations with China to 30-year lows, and Japan's looming pensions and welfare crisis.
Japan has millions of pensioners and a very low birth rate, meaning fewer workers to support pensions and health. "The future collapse of this system is the most important issue in this country, not whether the post offices are privatised or not," said the DPJ leader, Katsuya Okada, in a weekend debate.
There are some signs that the DPJ's message is hitting home: a weekend Kyodo poll found the party narrowing the gap with the LDP by five percentage points; 42 per cent of respondents said their main concern was social welfare policies. But many wonder whether Mr Okada, an earnest but dull former trade bureaucrat, has the political skills or charisma to topple the Koizumi bandwagon.
Either way, Mr Koizumi, who has proved himself an unexpectedly shrewd and resilient operator since taking office, has broken the mould of Japanese politics. "I think this race marks a turning point for Japanese elections," Professor Jiro Yamaguchi of Hokkaido University told the Japan Times.Reuse content