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