TOKYO, Sept. 7 (Xinhua) -- Japanese Prime Minister Shinzo Abe filed his candidacy for the ruling Liberal Democratic Party (LDP) leadership election slated for Sept. 20 on Friday and will go head-to-head with former Defense Minister Shigeru Ishiba.

The ruling party's election will essentially decide not just the LDP's leader but also who will go on to serve as the nation's prime minister.

If Abe, 63, who is most likely to win the race with five out of seven intraparty factions supporting him, does so, his third-term at the helm will all but ensure he becomes the longest-serving prime minister.

The official campaigning for the LDP leadership was scheduled to begin on Friday, but both candidates have decided to postpone campaigning activities for a few days due to the deadly earthquake that rocked Hokkaido on Thursday.

Abe, who has held the top spot since 2012, was reelected for his second consecutive three-year term in 2015.

Former Defense Minister Shigeru Ishiba, 61, officially declared his candidacy on Aug. 10 and has since said that if Abe were allowed to run unopposed again, it would undermine democracy here.

Ishiba, 61, a lower house member, has in the past been outspoken about Abe and his policies and this will be the third time the veteran politician will attempt to win the ruling party's top post.

Abe, however, is widely believed to win his third three-year term as LDP president and in doing so becoming the longest-serving prime minister.

But Ishiba, who has the support of a powerful LDP faction and has held key posts in the party including defense minister, LDP secretary general and minister in charge of revitalizing local economies, may be able to take advantage of slumping support rate for Abe and his Cabinet following a slew of cronyism and document tampering scandals.

While many believe Abe's success will be a foregone conclusion, some believe that Ishiba's "honest and fair" politics may be a refreshing change from Abe who has long been caught up in cronyism scandals.

Ishiba has also been a vocal critic of Abe's plans to revise Japan's Constitution and actually rewrote his own constitution revision proposal in February.

While acknowledging some of the positives of "Abenomics", Ishiba is also a proponent of introducing more methods to bolster regional economies.

Ishiba, in light of the government's tardy handling of a spate of recent natural disasters, has proposed to set up a new ministry dedicated to taking charge of disaster prevention.