TOKYO: Japan's ruling bloc is headed for a hefty win in a July 10 upper house election, and along with like-minded allies could even get enough seats to open the path to revising the postwar, pacifist constitution for the first time, surveys showed on Friday.

Prime Minister Shinzo Abe's Liberal Democratic Party (LDP) also has a shot at the 57 seats needed to obtain a majority on its own for the first time since losing control of the chamber in 1989, the Nikkei business daily and other newspapers said.

Abe is casting the election for half the seats in the 242-member chamber as a referendum on his decision to delay a planned hike in an unpopular sales tax and his "Abenomics" recipe of hyper-easy monetary policy, spending and reform, amid growing doubts whether the strategy is working.

Abe's coalition is in no danger of losing power in the election but needs a solid victory to keep his party's lawmakers in line and perhaps stay on another three years after his tenure as LDP president ends in 2018.

The premier has played down his goal of revising the constitution, which conservatives see as a humiliating symbol of Japan's World War Two defeat but which admirers consider the source of the country's peace and democracy.

A Mainichi newspaper poll showed 45 percent opposed revision versus 36 percent in favour.

The opposition Democratic Party and three small parties, backed by grassroots groups, are trying to keep the ruling camp and its allies from winning the two-thirds needed to begin the process of revising the constitution.

Revision requires a two-thirds majority in the upper house and the lower chamber, where the ruling bloc already has such a "super majority", and a majority in a public referendum.

The LDP and its junior partner, the Komeito party, were on track to exceed their target of 61 seats, or a majority of those up for grabs, the Nikkei said. The Democrats, who have struggled to regain trust after a rocky 2009-2012 rule, looked likely to win around 30 seats, the surveys suggested.

Other newspapers' polls showed similar results, although up to 35 percent of voters were undecided. "If the opposition is going to do well, it will have to depend on those voters whom the polls cannot count - the undecided, the unaffiliated, etc.," said Chuo University political scientist Steven Reed.