Factbox: Japan's upper house election: how it works

TOKYO (Reuters) - The election for the Japanese parliament's upper house on July 11 could determine how smoothly the ruling Democratic Party can forge ahead with policies to cut the country's huge public debt.

The Democrats, who swept to power last year, have a huge majority in parliament's more powerful lower house and will run the government whoever wins the upper house poll, but the party needs an upper house majority to enact laws smoothly.

Below are key facts related to the upper house election:

* The upper house is the less powerful of parliament's two chambers. It lacks the authority to select a prime minister, and budgets and treaties can be enacted without its approval.

But the chamber can reject other bills approved by the lower house, and the legislation can then only be enacted by a two-thirds majority of the lower chamber. The Democrats and their coalition ally, the People's New Party (PNP), currently fall short of two-thirds of seats in the lower house.

* The upper house has 242 seats and its members serve for six years, with an election for half the seats every three years. The last upper house election was in July 2007, when the Democrats won 60 seats, becoming the party with the biggest number of seats.

* Of the 121 seats at stake this time, 73 are for prefectural or provincial constituencies, each with one to five seats, where candidates with the most votes win. The multi-seat districts make the election outcome hard to predict, since parties can choose to run more than one candidate, dispersing votes.

The remaining 48 seats are from a nationwide proportional representation bloc in which voters choose either a candidate or a party, and seats are allotted based on the total number of votes cast for parties and their candidates.

* At present, the Democrats rely on the People's New Party (PNP) for a coalition majority in the upper house. The two-party coalition needs to win 56 of the 121 seats being contested for a combined majority including one affiliated independent.

The Democrats need to win 60 seats for a majority on their own. If they fall short, they are likely to keep their coalition with the PNP but may also woo other parties. Possible partners include the Buddhist-backed New Komeito and the tiny Your Party.

* Past prime ministers, including Ryutaro Hashimoto in 1998, have resigned to take responsibility for their party's poor showing in upper house polls, although there is no legal requirement to do so. Prime Minister Naoto Kan could face pressure from the Democrats to step down should they fare badly.

(Reporting by Chisa Fujioka; Editing by Sanjeev Miglani)