The four-way contest for party president, set for Wednesday, is also the first election with two female candidates in a country where women are vastly underrepresented in politics.
Because the party controls a majority in parliament, its new leader is likely to win the general election for prime minister in November.
Whoever wins, the party desperately needs new ideas to quickly turn around plunging public support ahead of lower house elections coming within two months, observers say.
Unusually, two women — conservative Sanae Takaichi and more liberal Seiko Noda — are competing against front-running Taro Kono, the vaccinations minister, and former Foreign Minister Fumio Kishida.
Little change is expected in key diplomatic and security policies under the new leader, said Yu Uchiyama, a political science professor at the University of Tokyo.
Suga is leaving only a year after taking office as a pinch hitter for Abe, who suddenly resigned over health problems, ending his nearly eight-year leadership, the longest in Japan’s constitutional history.
