The Liberal Democratic Party's Shigeru Ishiba said Thursday he is resigning as head of his faction following a poor showing in last month's party leadership race against Yoshihide Suga, who is now prime minister.
The former defense minister leads a faction with 19 members, formed in 2015 to support his bid to become prime minister.
"I could not live up to their expectations, and so I must take responsibility," he told reporters after announcing the decision.
A vocal critic of Suga's predecessor, Shinzo Abe, Ishiba has made four unsuccessful attempts to become LDP president. While he enjoyed solid support from the party's rank-and-file members in the previous two races, the most recent poll put him a distant third behind Suga and former Foreign Minister Fumio Kishida.
Popular among the public but lacking support from fellow lawmakers, Ishiba has not made clear whether he intends to run in the next LDP leadership race in September next year.
Arrangements have been made to have former Environment Minister Ichiro Kamoshita replace Ishiba as faction chief, though some members could urge him to reconsider, according to the sources.
Speaking during a radio program on Wednesday, the 63-year-old said he felt a "strong sense of responsibility" for possibly turning those who supported his latest run into "collateral damage."
Hailing from Tottori Prefecture, where he has held a seat in the House of Representatives since 1986, the banker-turned-politician previously served as minister of defense, minister for farm and regional revitalization as well as LDP secretary-general and policy chief.