Four candidates in the Constitutional Democratic Party of Japan's leadership election said Sunday that the opposition party will challenge all members of the ruling Liberal Democratic Party involved in a slush funds scandal in the next general election.

The four CDP lawmakers, speaking in a televised debate, said their party will field candidates in the next House of Representatives election to challenge all LDP members involved in the scandal.

The four are former Prime Minister Yoshihiko Noda, 67, former CDP leader Yukio Edano, 60, current party leader Kenta Izumi, 50, and Lower House member Harumi Yoshida, 52, who on Saturday filed their candidacies in the party's Sept. 23 leadership election.

Noda said that the punishments meted out by the LDP to its members involved in the scandal are lenient.

"The people can't give a penalty without a candidate," Noda said. "We must make every effort to work together with other opposition parties" to put up rival candidates, he said.

Edano said, "The biggest challenge is to defeat lawmakers involved in the slush funds scandal." He added, "I think that everyone (from other opposition parties) will understand this."

Izumi said he wants the CDP to challenge all lawmakers involved in the scandal. Yoshida said such lawmakers should face the judgment of voters squarely.