TOKYO (Reuters) - Japanese Prime Minister Shinzo Abe's ruling Liberal Democratic Party (LDP) will likely gain more than twice as many votes as the biggest opposition Democratic Party in the July 10 upper house election, newspaper surveys showed on Monday.

Abe has delayed a sales tax increase by 2-1/2 years to October 2019, and said the LDP, along with its junior coalition partner the Komeito party, aimed to win a majority of the 121 seats to seek a mandate for his decision.

The Democratic Party, however, said a constitutional revision should be a major issue for the election as Abe aspires to revise the U.S.-drafted, pacifist constitution.

In a poll by the Asahi Shimbun daily, 38 percent of those surveyed said they wanted to vote for the LDP in the proportional representation seats, compared with 15 percent for the Democratic Party.

A similar survey by the Yomiuri Shimbun daily showed that 35 percent of respondents intended to vote for the LDP, while 12 percent for the Democratic Party.

Abe said in January the LDP wanted to win a combined two-thirds majority with Komeito and like-minded opposition parties at the July election to enable him to revise the constitution, although he has said in recent weeks that it is premature to discuss a constitutional revision as an election issue.

A formal amendment of the constitution requires approval by two-thirds of both houses of parliament and a majority in a referendum. Abe's ruling bloc already has a two-thirds majority in the lower house.

In an effort to prevent the ruling bloc and two opposition parties open to constitutional revisions from gaining a two-thirds majority of the upper chamber, the Democrats and three smaller parities have agreed to put forward unified candidates for the 32 single-seat constituencies for the July vote.

(Reporting by Kiyoshi Takenaka; Editing by Michael Perry)