A clear Democratic Party win would end 54 years of almost unbroken rule by the Liberal Democratic Party (LDP) and break a deadlock in parliament, where the opposition and its allies control the less powerful upper chamber and can delay bills.
A big victory would mean the Democrats would have to pay less attention to their small allies on the left and the right, making policy formation easier. Financial markets would also welcome a decisive election outcome.
The Mainichi newspaper, based on a detailed survey, said the Democrats appeared set to win a two-thirds majority in the lower house, a prediction in line with other media polls this week.
The Yomiuri newspaper said while some LDP candidates were catching up with their Democratic rivals, this was unlikely to change the trend much, adding that around 20 percent of voters were still not sure who they would vote for.
Analysts have said the tide was unlikely to turn in the LDP's favor at this late stage.
The Democrats have promised to focus spending on households, cut waste and wrest control of policy from the hands of bureaucrats. But their pledge not to raise the 5 percent sales tax for the next four years has raised concerns about further inflating Japan's already huge public debt.
Japan's jobless rate hit a record high of 5.7 percent in July and deflation deepened as household spending fell 2.0 percent from a year earlier.
