TOKYO - The starting gun was fired on Friday (Sept 7) in the race to decide who will be Japan's next prime minister, but there were no raucous brickbats nor fiery debates as election campaigning has been put on hold for the weekend because of the deadly Hokkaido quake.

Prime Minister Shinzo Abe and one-time defence minister Shigeru Ishiba were formally nominated to compete in an internal Liberal Democratic Party (LDP) election on Sept 20 that will decide who will be its President for the next three years.

As the LDP is Japan's ruling party, the winner will be prime minister until 2021.

But with Japan reeling from a powerful typhoon and earthquake this week, campaigning will kick off only on Monday with a news conference.

An election debate at the Japan National Press Club scheduled for Saturday (Sept 8) has also been postponed to next Friday (Sept 14), after Mr Abe returns from a scheduled visit to Vladivostok in Russia for the three-day Eastern Economic Forum that starts on Tuesday.

Mr Abe, 63, is widely expected to win the contest, having secured the backing of five of the party's seven factions that collectively account for about 70 per cent of the LDP's Members of Parliament.

Mr Ishiba, 61, an MP representing Japan's least-populated prefecture of Tottori, is however seen as a salt-of-the-earth leader with popular appeal among the local and regional chapters.

Both men had squared off in the Sept 2012 LDP election when the party was in the opposition to the then-Democratic Party of Japan. Ultimately, Mr Abe edged out Mr Ishiba owing to his strong support among the LDP's lawmakers.

It paved the way for him to be Prime Minister when the LDP won a national election held three months later, in Dec 2012.

Acknowledging Mr Ishiba's strong support in the rural areas, Mr Abe named him the party's secretary-general from 2012 to 2014, before appointing him the Minister for Regional Revitalisation from 2014 to 2016.

Party rules had, until a revision last year, capped the maximum tenure for LDP presidents to two consecutive three-year terms. Mr Abe was elected unopposed in 2015.

Mr Ishiba, with his sights set on succeeding Mr Abe, refused a ministerial post in the Cabinet in 2016 so that he could be more vocal in challenging Mr Abe's brand of politics.

But he was caught offguard by the rewriting of the party rules, the Mainichi Shimbun reported this week, to let Mr Abe be re-elected and become Japan's longest-serving prime minister.

The newspaper added, quoting party insiders, that Mr Abe has consolidated power to such an extent that some in the LDP are hesitant to back Mr Ishiba, out of fear that they may be sidelined from important positions.

Another three years as Japan's Prime Minister will give Mr Abe the time he needs to push through social security reforms - including raising the retirement age to 70 - and his desired revision of the pacifist Constitution.

Pro-Abe factions are calling for a proposal to be submitted to Parliament this year for debate. Mr Ishiba, too, wants the supreme law revised, but said it should not be rushed without enough public understanding.

Any constitutional change will need the support of two-thirds of the lawmakers in both the Lower and Upper House - numbers the LDP currently have although the next Upper House poll will be held in July (2019). It will also need a majority in a public ballot.

Mr Abe will also campaign on his track record as Japan's leader since December 2012, pointing to the gradual improvement of the economy and the country's growing international clout on issues such as free trade and the rule of law.

Mr Ishiba has styled himself as Mr Abe's fiercest critic, promising to uphold "honest, fair" politics in what some in the LDP has branded as a personal attack on Mr Abe.

It was prompted by two favouritism scandals that have bedevilled Mr Abe for more than a year, damaging his public approval ratings. Even now, media polls show most people do not buy his explanation that he did not know about the scandals.

Yet, polls last weekend show that the general public slightly prefer Mr Abe to Mr Ishiba. A poll by broadcaster TBS show 40 per cent prefer Mr Abe to Mr Ishiba's 39 per cent, as the LDP's next leader.

A separate poll by the Mainichi daily show 32 per cent prefer Mr Abe, to Mr Ishiba's 29 per cent. Another 28 per cent, however,say neither is fit for the job.