Prime Minister Shinzo Abe has achieved much of his conservative security agenda since taking office in 2012, but unless he can revive his flagging popularity, his goal of revising the pacifist Constitution is likely to elude his grasp.

Failure to achieve that goal by the 2020 target he announced three months ago will erode Abe's already weakened clout, dimming his chances of becoming Japan's longest-serving prime minister, lawmakers in his own Liberal Democratic Party said.

"Abe is filled with a desire to do this. He thinks revising the Constitution is his greatest mission as a politician . . . but can he really?" LDP lawmaker Katsuei Hirasawa said.