A triumph in the Sunday balloting for the Liberal Democratic Party (LDP) would be a personal victory for Koizumi, who has put his job on the line by making the vote a referendum on his two-decade quest to privatise Japan's mammoth postal service.
Newspaper polls on Friday show the LDP - which has ruled Japan almost continuously since 1955 - is on its way to clinching most of the 480 seats up for grabs in the powerful lower house.
The opposition Democrats were expected to finish a distant second.
Combined with seats held by ally New Komeito Party, such a victory would grant Koizumi - a solid US ally - a mandate for his plan to split up and sell off Japan Post's delivery, savings and insurance services, creating the world's largest private bank.
An LDP win would also bolster Koizumi's standing as one of the most dynamic Japanese political personalities of the postwar era.
After losing a vote on postal reform in the upper house on 8 August, Koizumi - prime minister since 2001 - took a bold gamble by dissolving the lower house and calling snap elections.
The campaign that followed has been the most riveting in recent memory in Japan, as Koizumi purged 37 anti-reform lawmakers from his party and drafted celebrity candidates, including a former beauty queen and an internet mogul, to run as "assassins" against his enemies.
Victory would add to Koizumi's
standing as a dynamic leader
The drama has fascinated a country long accustomed to highly scripted campaigns without burning issues or distinctive personalities.
One poll forecast turnout would be 75%, a hefty jump over the 60% participation in the last lower house elections in 2003.
"This election is definitely different," said Masaru Mori, a retired worker from an IT company. "It is an election that may break away the established order - it's different from the halfhearted elections of the past."
Koizumi has also been boosted by the economy, which is showing strong signs of emerging from more than a decade of stagnant growth. The stock market is rallying, the job market is the best in years, bad debts at banks have dwindled.
Still, it is uncertain whether a Koizumi victory would really usher in the era of reform that he is promising.
Many factions in the LDP are opposed to deep change, and the prime minister says he will step down at the end of his term in a year, leaving the future course of the LDP unclear.