Nicknamed the "Lion Prime Minister" (ライオン宰相, Raion Saishō) due to his dignified demeanor and mane-like hair,[1] Hamaguchi served as leading member of the liberal Rikken Minseitō (Constitutional Democratic Party) during the "Taishō Democracy" of interwar Japan.

He initially survived an assassination attempt by a right-wing extremist in 1930, but died about nine months later from a bacterial infection in his unhealed wounds.

Hamaguchi graduated from the Law College of Tokyo Imperial University in 1895 and began his career as a bureaucrat in the Ministry of Finance.

After the collapse of the administration of Tanaka Giichi in June 1929, Hamaguchi was selected to become Prime Minister of Japan and formed a cabinet based largely on Minseitō party members, which supported domestic economic reforms over overseas military adventurism.

However, his measures to help stimulate exports, such as maintaining the Japanese yen on the gold standard, proved disastrous.

The opposition Rikken Seiyūkai joined forces with the vocal anti-Treaty faction within the Imperial Japanese Navy to accuse Hamaguchi of infringing of the military's "right of supreme command" as guaranteed under the Meiji Constitution.

[6] Hamaguchi's initial popularity quickly waned, and he fell victim to an assassination attempt on 14 November 1930 when he was shot inside Tokyo Station by Tomeo Sagoya, a member of the Aikokusha ultranationalist secret society.

The Seiyūkai immediately attacked the government on the grounds that the Prime Minister was not physically present, and that Shidehara was not even a member of the Minseitō.

When Shidehara further created an uproar with a comment concerning Emperor Hirohito's support of the London Naval Treaty, the Seiyūkai refused to participate in budget deliberations until Hamaguchi could attend.