A month ago he was appointed secretary-general of the powerful Liberal Democratic Party. But the political winds have changed again, and he looks unlikely to survive beyond September. At 68, time is running out for the unlucky Mr Mitsuzuka and he has few cards left to play. So how about this? According to a recent book, Mr Mitsuzuka is the reincarnation of a famous samurai, a blood relative of the Buddha, and the man destined, if he is given the chance, "to create a government that receives divine protection from the Heavenly Realm".
The claims are not made by Mr Mitsuzuka, but by a curious religious group called the Institute of Research in Human Happiness (IRHH).
Even by the standards of Japanese "new religions", the Institute is odd. Its founder, 39-year-old Ryuho Okawa, is a former businessman who founded the sect in 1986 allegedly at the request of Buddha, Christ, Confucius and Albert Einstein, of whom he is the reincarnation. Today the group claims a membership of more than 2 million. It is rich, with income from donations, tapes, and sales of Mr Okawa's books independently estimated at pounds 30m a year.
Its theology mingles Buddhism, Christianity and personal contributions from the founder's divine forebears, with whom he converses regularly.
Mr Okawa has always had a lively interest in international affairs, and in his previous books he predicted an apocalypse in which Japan would triumph over Russia and America. However, at a rally in Tokyo attended by 50,000 followers, he called for closer ties with the US.
"We should erect the tower of light at the top of the Japanese government ... I recommend Hiroshi Mitsuzuka, a full member of the IRHH to be the next prime minister, not for ourselves but for the world and for the Earth", he said.
Mr Okawa then revealed Mr Mitsuzuka's previous incarnations as the Buddha's uncle 2,000 years ago, and as a 17th-century samurai famous for killing a tiger. The book containing Mr Okawa's address, The Right Way for Japan to be Reborn, is a bestseller and will be published in English this month.
For a respected politician to associate himself with such eccentrics would be remarkable at any time, but is truly striking in Japan today. Next month the trial will begin of another cult guru, Shoko Asahara, whose Aum Shinri Kyo sect is believed to have murdered 12 commuters in the nerve-gas attack on the Tokyo subway in March.
After Mr Asahara's arrest in May, there was all-party agreement that Japan's Religious Corporation Law, which grants tax breaks to 185,000 registered religious groups, should be tightened up. A bill is to be presented to the Diet in a fortnight. But comprehensive revision of the law looks unlikely. Mr Mitsuzuka cites the difficulties of squaring a stricter law with the constitution, which guarantees freedom of religion, while insisting on the separation of religion from the state. But the more likely reason is the huge influence wielded by religious groups on politics.
Japanese religions offer two vital commodities to the parties - money and votes. The latter has been in short supply recently. Elections for the Upper House of the Diet in August saw the lowest turnout in Japanese history. But the opposition Shinshinto (New Frontier Party) made remarkable gains thanks, it is believed, to the support of Soka Gakkai, a Buddhist lay organisation supported by 8 million households.
Apart from the personal endorsement of the LDP secretary-general by the Institute of Research in Human Happiness, the LDP has links with Rissho Kosei-kai, a neo-Buddhist group of 6.5 million members. With fewer than half those eligible to vote turning out, these are significant numbers.
Mr Mitsuzuka is reluctant to go into detail about his membership of IRHH and his former lives. "It's a private matter," he says, "my personal freedom of religion." He stresses the importance of "distinguishing between legitimate groups and those liable to commit criminal acts". Although a government advisory committee has been convened, 11 of the 15 members represent religious groups. Their recommendations willcontain nothing to trouble Christ, Confucius or Mr Okawa.