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.

[6][7] He attempted to run for the position again in 2014, without the backing of the DPP, which expelled him for mounting an independent campaign.

[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.