The big political parties are concerned about voter turnout for Sunday's Upper House election and its impact on the eventual makeup of the Diet.

Ruling and opposition parties believe interest in the election is weakening, as media reports have suggested the ruling coalition, led by Prime Minister Shinzo Abe's Liberal Democratic Party, is headed for a comfortable victory.

They predict voter turnout will fall below the 57.9 percent seen in the previous Upper House election in 2010.