Azuma Koshiishi, chairman of the Democratic Party of Japan's caucus in the Upper House, threatened Thursday to submit a censure motion against Prime Minister Yasuo Fukuda and warned him against fighting the opposition camp in the chamber out of spite.

"(The opposition parties) can submit a censure motion against the prime minister, and what is more, we can approve it" in the Upper House, Koshiishi said.

He also accused Fukuda of cowardice by pointing out that he hadn't dared run in last year's Liberal Democratic Party presidential election against Shinzo Abe for fear of losing, as Abe was very popular at that time and assured of winning the race. After Abe's abrupt resignation in September, most of the LDP factions moved quickly to support Fukuda this time round.