By Linda Sieg and Izumi Nakagawa

TOKYO (Reuters) - Prime Minister Shinzo Abe's cherished goal of revising Japan's pacifist constitution has become more difficult to achieve after a plunge in his popularity and the erosion of public trust, a ruling party lawmaker said on Wednesday.

Support for Abe has plummeted to its lowest since he surged back to power in 2012 with a conservative agenda of reviving traditional values and loosening constraints on the military that centers on revising the U.S.-drafted post-war constitution.

In May, Abe made a surprise proposal to revise the charter's war-renouncing Article 9 by 2020 to clarify the ambiguous status of its military, known as the Self-Defense Forces, by 2020.

Meeting that deadline would mean adopting an amendment in parliament next year, since pro-revision forces in the lower house are likely to lose their super-majority in an election that must be held by late 2018.

Amendments need the approval of two-thirds of both chambers and a majority in a referendum.

"There is no change in the goal toward which we are working but greater efforts are needed now to achieve that goal," Hajime Funada, deputy head of a ruling Liberal Democratic Party task force on constitutional reform, told Reuters in an interview.

"Rather than a matter of 'yes' or 'no' toward revising Article 9 itself, trust and expectations toward Prime Minister Abe, who is advocating it, have fallen sharply," Funada said, adding that the LDP's junior coalition partner, the Komeito party, had also grown more cautious about amending the charter.

Amending Article 9, which renounces the right to wage war as a way to settle international disputes, is a divisive issue in Japan.

Supporters of the article see it as the foundation of post-war democracy but many conservatives see it as a humiliation, imposed after defeat in World War Two.

Amending the article would also raise concern in China and South Korea, where bitter memories of the conflict run deep.

'IN A BIND'

Abe's proposal would be to retain the two clauses of Article 9 that renounce the right to wage war and ban maintenance of air, land and sea forces, while adding a clause legitimizing the SDF.

The impact of that change is hotly debated. Proponents say it would merely inscribe existing policies in the constitution, while critics worry it would open the door to a bigger role for the military overseas.

Abe's popularity has been battered by suspicions of scandal over favoritism for a friend's business and by the perception among many voters that he and his aides have grown arrogant.

The prime minister is set to reshuffle his cabinet next month to try to revive his sagging support, but Funada said the impact of personnel changes would probably be limited.

"Unless he changes his attitude and his mindset, things will not improve," Funada said.

The appearance that Abe is hurrying to amend the constitution while he himself is in office was making the party task force's job harder, Funada said.

Abe is keen to achieve his goal in part because it eluded his grandfather, a conservative who had to resign as prime minister in 1960 due to a public furor over a U.S.-Japan security pact.

Until recently, Abe was favored to win a third three-year term as LDP leader, and hence premier, when his current term expires in September 2018, but that has become less certain.

"His feeling of wanting to try to revise the constitution while premier and if possible, succeed, is taking precedence and that has begun to be obvious," Funada said.

"We're in a bind."

(Reporting by Linda Sieg; Editing by Robert Birsel)