A typhoon is expected to lash Japan with heavy rains on Sunday, potentially weighing on turnout as millions of voters head to the polls in the world's third-biggest economy.
Prime Minister Shinzo Abe appealed to voters to cast their ballots early as Typhoon Lan moves towards Japan bringing driving rain across the country on election day.
"It's rare to see typhoon rains over such a large swathe of the Japanese archipelago in October," Eiju Takahashi, an official with the Japan Meteorological Agency, told AFP.
Only the northern island of Hokkaido is expected to be spared the downpour on Sunday, added Takahashi.
Abe himself cast his vote on Wednesday in Tokyo, telling reporters that the weekend election "would decide Japan's future" and urging voters to cast ballots early in anticipation of bad weather.
Turnout has declined to below 60% in the last two general elections. The last election in December 2014 saw a record-low rate of 52.66%.
"If it rains on Sunday, the turnout rate will not rise and that would benefit the ruling bloc," said Mikitaka Masuyama, political scientist at the National Graduate Institute for Policy Studies.
Swing voters tend to abstain on rainy days while staunch supporters of the ruling Liberal Democratic Party and its coalition partner Komeito show up to voting stations whatever the weather, observers say.
"If the turnout rate hits a new record low, that would probably mean the ruling bloc maintains a two-thirds majority" said Hidenori Suezawa, financial and fiscal analyst at SMBC Nikko Securities.
That is significant as Abe's conservative LDP needs a two-thirds majority in both houses of parliament in order to propose changes to the country's constitution.
"On the other hand, if the turnout rate rises to the 60% level, new (opposition) parties may make a leap forward," he said.
The key question is how heavy the rain is, said Suezawa.
"If it is cloudy or there is light drizzle, turnout may rise as families may abstain from outings ... but if rains heavily, it could weigh on the turnout rate," he said.