TOKYO, Aug. 21 (Xinhua) -- Japan's ruling Liberal Democratic Party (LDP) decided on Tuesday to hold its leadership election on Sept. 20, in what is likely to be a two-horse race between between Prime Minister Shinzo Abe and 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 next prime minister.

If Abe, 63, who is currently favorite 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.

Official campaigning for the LDP leadership is slated to begin on on Sept. 7 and Abe is likely to officially declare his candidacy next week, sources close to the party have said.

Abe has been unopposed and was reelected for his second consecutive three-year term in 2015.

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

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

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 a 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 for some 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.

Ishiba, known as a security hawk, said recently that the constitutional issue is a thorny one, and any amendments shouldn't be rushed amid a continued lack of public understanding.

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

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

Ishiba told a press briefing Tuesday that the LDP should make the buildup and the voting process conducive to being able to fully and publicly discuss all candidates' viewpoints on all aspects of policy.