The Democrats picked the bouffant-haired Hatoyama to replace his scandal-hit predecessor in a May leadership race, seeing him as best able to hold the sometimes fractious party together.

He was not, however, the most popular candidate with the public, who saw him as being under the shadow of previous party leader Ichiro Ozawa.

Hatoyama attracts more support than Prime Minister Taro Aso in opinion polls, but many voters say they see neither as suitable to be premier.

"His best quality is that he's not Aso," said Jeff Kingston, professor of Asian studies at Temple University's Tokyo campus. "He's a bit of a cipher. He's prominent, but he doesn't leave a strong impression."

Aso, the grandson of a former prime minister, has been criticized as out of touch with ordinary Japanese because of his wealthy background.

But Hatoyama, a Stanford University PhD once nicknamed "the alien" for his prominent eyes, hails from an even wealthier family of industrialists and politicians. His mother's father founded Bridgestone Corp, one of the world's largest tire makers.