Campaigning began Wednesday for the leadership contest of the second-largest opposition party, with contenders tasked with boosting the party's support base and cooperating with other members of the opposition camp.

The Sept. 4 election to pick the head of the Democratic Party for the People is expected to be a two-horse race between two Lower House members, Yuichiro Tamaki and Keisuke Tsumura, who both registered their candidacies on Wednesday.

Tamaki, a four-term member of the House of Representatives, currently co-heads the party. He will be challenged by Keisuke Tsumura, a six-term member of the Lower House. The new leader will serve until September 2021.