As Japan looks ahead to the ruling party's presidential election, which will effectively pick the country's new prime minister, official campaigning kicked off Monday in the race to choose the leader of a soon-to-be-formed major opposition party.

Yukio Edano, 56, who heads the Constitutional Democratic Party of Japan, and Kenta Izumi, 46, who is the policy chief of the Democratic Party for the People, have filed their candidacies for the Thursday election for the president of the new party.

The country's two largest opposition forces — which, along with some independent politicians, total 149 Diet members — will merge on Sept. 15 in the hope of mounting a united front against the ruling Liberal Democratic Party and its coalition partner Komeito.