A cofounder of the Democratic Progressive Party,[1] Tsai served on the National Assembly from 1992 to 1997, and was the DPP caucus leader throughout his term.

Tsai was nominated as the DPP candidate for the deputy speakership, and lost to Shieh Lung-sheng.

[10][11] Soon after leaving office, Tsai was caught in the Chu Mei-feng sex scandal, during which he was represented by attorney Hsu Wen-bin.

[15] Prosecutors believed that Tsai and spiritual advisor Kuo Yu-ling asked a detective agency about hidden cameras.

[16][17] Tsai was questioned by the Taipei District Prosecutors' Office in January 2002,[18][19] and indicted on 7 February.