Earlier this year, Rep. Cho Jun-hyuk of the ruling Grand National Party (GNP) posted on his Web site the names and personal information of elementary and secondary school teachers who had joined the Korean Teachers and Education Workers' Union (KTU).
Nine other GNP members followed suit in a show of support for Cho.
Seoul courts later ruled that the disclosure was illegal and ordered Cho to pay the KTU 150 million won in restitution.
"We decided to file the damage suit because the nine lawmakers have not apologized for their conduct and insisted that what they did was right, even when the court said that it was illegal," the KTU said.
The nine lawmakers are Chin Soo-hee, the new health and welfare minister, along with Chung Doo-un, Kim Yong-tae, Cha Myeong-jin, Jeong Tae-keun, Chung Jin-suk, Chang Je-won, Kim Hyo-jae and Park Jun-seon.
The KTU said that it would seek 1.2 billion won in damages, about 200,000 won each for the 5,864 members whose names were disclosed by the lawmakers.
It also said the union would seek means to confiscate the 150 million won from Cho, who has refused to pay it.
The controversy with the GNP had put the 400,000-member KTU in deeper confrontation with the Lee Myung-bak administration whose key policies drew criticism and opposition from the union.
Those who support the disclosure say it promotes the rights of parents and students to know, while critics said it arms the schools and the government with the means to seek out and reprimand teachers with liberal tendencies.