Goshi Hosono, a former secretary-general of the Democratic Party of Japan, announced he will run in the party's leadership race on Jan. 18.
"I've decided to take on the challenge of running in the election," Hosono, who served as nuclear disaster minister and environment minister when the DPJ was in power, told reporters Wednesday after executives of the party decided on the election date.
Outgoing DPJ President Banri Kaieda said he would resign as leader after losing his seat in Sunday's House of Representatives election, in which the ruling coalition of the Liberal Democratic Party and Komeito retained a two-thirds majority.
Hosono said no force in the Diet is strong enough to challenge the administration of Prime Minister Shinzo Abe, who heads the LDP. "The DPJ's in an extremely tough spot but the opposition camp must rebuild to protect Japan's democracy. I believe only the DPJ can play a pivotal role (for such efforts)," he said.
The DPJ, the Diet's largest opposition party, has been struggling to regain the support of voters who were dissatisfied with its performance when it was in power for the first and only time from 2009 to 2012.
Other DPJ members are pushing for former leader Seiji Maehara, the current acting leader Katsuya Okada or Secretary-General Yukio Edano to take over.
Maehara met on Wednesday with about 10 lawmakers whom he is close with and some urged him to throw his hat into the ring. He told reporters afterward he has not made up his mind on the matter.
Okada did not comment on whether he will run in the presidential election, only telling reporters it is an "honor" to be recommended to head the party.
