By Linda Sieg

TOKYO (Reuters) - Campaigning for an election to Japan's upper house of parliament begins on Wednesday, as surveys give Prime Minister Shinzo Abe's ruling bloc the lead, despite doubts about his growth policies and desire to revise the post-war pacifist constitution.

Abe's coalition is in no danger of losing power in the election but he needs a solid win to keep his Liberal Democratic Party (LDP) lawmakers in line and perhaps stay on another three years after his tenure as LDP president expires in 2018.

Abe is casting the July 10 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.

"Our policies have had some success, but we are still only halfway to where we want to be," Abe said in a televised debate on Tuesday. "I want to accelerate Abenomics, shift up two or three gears and make sure the economy is growing."

Media surveys show about twice as many voters plan to vote for Abe's Liberal Democratic Party (LDP) as for the main opposition Democratic Party, but also show support for Abe and his party slipping amid growing doubt that his efforts to revive the economy are working.

Turnout is expected to be weak after hitting a record low of 52.6 percent in a 2013 upper house vote.

About 2.4 million Japanese aged 18 and 19 will be able to vote for the first time, but surveys suggest the turnout of this group will be lower than among their parents and grandparents.

The expected victory for the ruling bloc is thus more likely to be a vote of no-confidence in the opposition than a groundswell of support for Abe and his policies.

"Abenomics is not working out well and Abe's security policies are not popular. You'd think he'd be vulnerable," said Jeffrey Kingston, director of Asia studies at Temple University's Japan campus.

"But the opposition is weak and discredited, and even though they will be cooperating, I don't think it will make a big difference."

Some analysts, however, said the LDP could lose some of the 51 seats it held among those up for grabs, thanks to a historic move by opposition parties to join hands.

The Democratic Party and three smaller parties, including the Japanese Communist Party, are backing unified candidates in 32 single-seat districts, and have support from grassroots civic groups opposed to Abe's hawkish security policies and drive to revise the constitution.

But regaining public trust is a tough task for the Democrats after a 2009 to 2012 tenure many remember for infighting and unkept promises.

"The Democratic Party is the only opposition party that has experience governing and can learn from its mistakes and change," its policy chief, Shiori Yamao, told Reuters, adding that it was also drawing on the energy of women and the youth.

"We are becoming a party that cannot only take power but govern properly, so please believe in us again."

Abe has set a target for his coalition of winning a majority of the 121 seats being contested. [L4N19C1PI]

The premier has said the ruling bloc hopes to win a two-thirds majority with like-minded opposition parties to open the path to revising the postwar pacifist constitution, but has recently played down that target.

Surveys show a majority of voters see no need to change the charter, which conservatives see as an obstacle to beefing up defense and a humiliating symbol of defeat in World War Two.

(Additional reporting by Tetsushi Kajimoto and Stanley White; Editing by Clarence Fernandez)