A senior lawmaker of the opposition party Osaka Ishin no Kai expressed a willingness Tuesday to help Prime Minister Shinzo Abe build a force that can win more than two-thirds of the seats in the House of Councilors in next summer's election to initiate amendments to the Constitution.

The party "will cooperate positively," Secretary-General Nobuyuki Baba said at a news conference, referring to Abe's recent remarks that his ruling coalition is willing to cooperate with Osaka Ishin no Kai and other small opposition parties in his quest to revise the U.S.-drafted charter.

Speaking in an interview with NHK aired Sunday, Abe said it "would be difficult for the ruling parties alone to win two-thirds or more" of the 242-seat Upper House in the election likely to take place in July.