Kobayashi is a member of the ruling Democratic Party of Japan, and the arrests from the major support base will likely deal more damage to the administration of Prime Minister Yukio Hatoyama, which has been fighting off political funding scandals since taking power in the historic House of Representatives election Aug. 30 that removed the long-ruling Liberal Democratic Party.

The three union members are Hideki Osada, 50, acting chairman of the Hokkaido Teachers Union, its secretary general, Kentaro Koseki, 54, and accountant Takaaki Nambu, 52, the Sapporo District Public Prosecutor's Office said. The union is part of the Japan Teachers Union.

They allegedly donated a total of 16 million yen over four occasions between December 2008 and July 2009 for Kobayashi's campaign in violation of the Political Funds Control Law. Michiru Kimura, 46, the fourth person arrested, received the sum as chief accountant for the campaign, according to the prosecutors.

Among the campaign staff to Kobayashi, who was elected to her second Lower House term by the Hokkaido No. 5 constituency, acting chief Hirokazu Yamamoto, 60, is appealing a guilty verdict handed down last month by the Sapporo District Court.