Ms. Chiyomi Kobayashi, a former saleswoman for a bread company who was first elected to the Lower House by proportional representation in 2003, was touted as a symbol of the Democratic Party of Japan's massive win in the Aug. 30 Lower House election. She routed former Chief Cabinet Secretary Nobutaka Machimura of the Liberal Democratic Party in Hokkaido's No. 5 constitution with a margin of more than 30,000 votes. (Mr. Machimura managed to keep a Lower House seat through proportional representation.)

But on Monday she announced her resignation over a political fund scandal involving a labor union. The DPJ and labor unions, which maintain close ties, must learn a lesson.

On Monday, the Sapporo District Court handed a suspended prison sentence to Mr. Hideki Osada, acting chairman of the Hokkaido Teachers Union, and fined the union ¥500,000 for providing ¥16 million to her campaign on four occasions between December 2008 and July 2009 — in violation of the Political Funds Control Law, which bans labor unions from donating to a campaign office. Mr. Osada and the union accepted the ruling.