Shinzo Abe is poised to win a third term as head of his political party on Thursday, putting him on track to become Japan's longest-serving prime minister and realize his dream of reforming the constitution.

Polls show Abe, 63, is expected to romp to victory in a two-horse race for the leadership of his conservative Liberal Democratic Party (LDP) that will effectively hand him three more years as PM.

His only challenger is former Defense Minister Shigeru Ishiba, a hawkish self-confessed "military geek" who also wants to make controversial changes to the country's post-war pacifist constitution.

Only a few months ago, Abe's continued leadership at the helm of the world's third-biggest economy was less assured, as he became mired in a series of cronyism and cover-up scandals.

Ishiba has said he is running to "restore confidence in the government, which should be at the service of citizens" after Abe's administration became embroiled in simultaneous influence-peddling, cover-up and sexual harassment cases.

But Abe survived these problems -- despite a dip in popularity -- and recent surveys show he is likely to win two-thirds of the votes in the election, in which LDP lawmakers and party members are eligible to vote.