Japan's widening wealth gap has emerged as a key issue in a ruling party leadership contest that will decide who becomes the next prime minister, with candidates forced to reassess the legacy of former premier Shinzo Abe's "Abenomics" policies.

Under Abenomics — a mix of expansionary fiscal and monetary policies and a growth strategy deployed by Abe in 2013 — share prices and corporate profits boomed, but a government survey published earlier this year showed households barely benefited.

Mindful of the flaws of Abenomics, front-runners in the Liberal Democratic Party's leadership race — vaccination minister Taro Kono and former Foreign Minister Fumio Kishida — have pledged to focus more on boosting household wealth.