TOKYO, Sept 10 (Reuters) - Japan"s popular vaccination minister, Taro Kono, is set to announce his candidacy on Friday for leader of the country"s ruling political party and, by extension, its next prime minister. Kono"s decision to throw his hat in the ring for the leadership of the Liberal Democratic Party (LDP), left open by Prime Minister Yoshihide Suga"s announcement last week he would step down, takes the number of candidates to three. Kono appears to have an edge on former foreign minister Fumio Kishida and former internal minister Sanae Takaichi in the race. Nearly a third of respondents in a poll conducted by major domestic media last week said the Georgetown University-educated Kono, 58, was the most suitable to succeed Suga. The winner of the Sept. 29 vote of grass-roots party members and its lawmakers will lead it in the lower house election that must be held by Nov. 28. The LDP president is virtually assured the premiership, because the party has a majority in parliament"s lower house. Rank-and-file lawmakers are counting on the new leader to provide a jolt in popularity after Suga"s ratings hit record lows. Kishida is reasonably popular and can count on the support of his faction of the party, while Takaichi has supporters in the conservative flank, including influential former prime minister Shinzo Abe. The last unknown is whether the former defence minister Shigeru Ishiba, who is also well-liked among the regular members of the party, will decide to run on his own or throw his weight behind Kono. While Suga"s support tanked due to his haphazard handling of the coronavirus pandemic, Kono, who has been in charge of the rocky vaccination rollout, has stayed popular, particularly among the younger voters. This was thanks to his ability to go off script and directly reach out to the public through Twitter, where he has 2.3 million followers - a rarity in heavily scripted Japanese politics dominated by older men less adept with social media. Some in the LDP feel he is too young, given the average age since 2000 for premiers to take office is roughly 62. Their concerns also include his lone-wolf character in a land that runs on consensus, as well as an outspoken streak and occasional divergence from the standard line. Despite that reputation, Kono toed the line on Abe"s key policies when he served as both defence and foreign ministers in his cabinets. He has differentiated his conservative stances from those of his father, former chief cabinet secretary Yohei Kono, who authored a landmark 1993 apology to "comfort women", a euphemism for women forced to work in Japan"s wartime military brothels. Kono is due to announce his candidacy at 4 p.m. in Tokyo (7:00 GMT), according to his office.