TOKYO - Japanese Prime Minister Shinzo Abe is tipped to handsomely win another three years as chief of the ruling Liberal Democratic Party (LDP) on Thursday (Sept 20), in an election that has been fought on constitutional revision, the economy, and the need for clean governance.

Mr Abe, who turns 64 on Friday, has been Premier since December 2012. As the ruling party leader is usually prime minister, a victory over his rival, former defence minister Shigeru Ishiba, 61, will set him on track towards becoming Japan's longest-serving leader.

It will buy him three more years to achieve his long-held goal of amending the pacifist supreme law, as well as to pursue deeper economic reforms and to bolster social security measures in light of a fast-ageing society where one in every five people is aged 70 and above.

A poll by public broadcaster NHK on Tuesday (Sept 18) showed that Mr Abe has the support of 80 per cent of the LDP's 405 parliamentarians, who each get one vote. Another 405 votes are apportioned among the 1.04 million rank-and-file party cadres in local chapters.

Mr Ishiba, who represents Japan's least-populated prefecture of Tottori, sees regional revitalisation as his pet cause and argues that Mr Abe's economic policies have failed to uplift the rural areas. Even so, a Kyodo News poll showed that about 60 per cent of rank-and-file members intend to back Mr Abe.

This means the odds are likely to be stacked against Mr Ishiba, who was narrowly defeated by Mr Abe when they squared off in 2012. There was a walkover in 2015.

Party rules were rewritten last year to extend the term limit for party chief to three consecutive three-year terms, instead of two. Mr Abe has said this will be the last LDP election he will contest, effectively putting an end date to his tenure.

Analysts say that anything less than a landslide win may render Mr Abe a lame duck in his last term. While he stabilised a country that had six prime ministers from 2006 to 2012, when he took charge, the last year has been brutal for his government.

Heads rolled in the civil service, as the administration was hit by scandals including cronyism, data tampering, corruption and sexual harassment.

If Mr Abe wins, among his first orders of business will be a visit to the United States next week to attend the United Nations General Assembly. He is due to meet US President Donald Trump, with whom he has assiduously tried to build a friendship on the golf course but has constantly been blown off on trade.

He has also said that he will reshuffle his Cabinet - only appointed in August last year - early next month so that "reliable people" will be in place to lead Japan into a new era.

Emperor Akihito will step down from his throne on April 30 next year, closing the chapter on the current Heisei (achieving peace) era. Japan, too, will host the Group of 20 (G-20) leaders' summit next year, and the Tokyo Olympics and Paralympics in 2020.

Mr Abe has attained so much clout within the LDP that former rivals have thought it wiser to back him. They include former foreign minister Fumio Kishida, who declared he would stay out of the race, and internal communications chief Seiko Noda, who could not muster the 20 supporters she needed to run. While she previously voted for Mr Ishiba, she is expected to vote for Mr Abe this time round.

Political princeling Shinjiro Koizumi, who voted for Mr Ishiba in 2012, has kept mum over who he will support this time.

The fortnight-long hustings have been marred by allegations of foul play. Agriculture minister Ken Saito, who backs Mr Ishiba, has said that an Abe supporter has heckled him and told him to quit the Cabinet if he does not vote for the Premier.

The left-leaning Asahi Shimbun newspaper, too, has noted that Mr Abe was putting himself at the front and centre of disaster response after a devastating typhoon and earthquake earlier this month, announcing updates to death tolls and recovery measures when these are normally left to his top government spokesman Yoshihide Suga.

With the economy humming along, University of Tokyo political scientist Yu Uchiyama expects a "status quo bias" to prevail. He said: "When economic conditions are good, there is a tendency for voters to prefer maintaining status quo rather than take risks."

Even then, he expects constitutional revision to be the key challenge for Mr Abe, should he win a new term. While the LDP has numbers in both houses of Parliament, it needs a majority in a national referendum and public opinion is still mixed.

The pacifist supreme law, drafted by the US after World War II, stipulates that armed forces will "never be maintained". While this has been reinterpreted to allow a military for self-defence purposes, the Self-Defence Force (SDF) is arguably stuck in a grey zone.

Mr Abe argues that this is not tenable, and wants to codify explicitly in law that the SDF "protects the peace and independence of Japan". He wants the LDP to table proposals to Parliament by year-end. Mr Ishiba, while advocating a more hawkish revision, has urged caution in not steamrolling the process.

"In the event the proposal gets voted down by the Japanese people in a national referendum, there is a risk that Mr Abe might be forced to resign" given that this has been front and centre of his ambitions, Dr Uchiyama said.