Polls suggest Hatoyama may be vulnerable at the helm

As Japan's prime minister Yukio Hatoyama saw his approval rating dip from more than 70 percent to 48 percent, the lowest since he was inaugurated in September, speculation looms that the ruling Democratic Party of Japan (DPJ) might opt to replace him well ahead of Japan's Upper House election next July.

Winning the election is seen as the paramount goal for DPJ, said Wang Ping, scholar with Chinese Academy of Social Sciences Japanese Studies Institute, therefore, the party may try to get rid of any factor that may 'impair' its winning efforts.

However, a prime minister replacement would be "an extreme scenario", said Zhou Yongsheng, Japan studies professor with China Foreign Affairs University.