His father Hideo Yamahana was a labour activist who was arrested several times for alleged subversion during Sadao's early childhood in the wartime period.

In July of the same year the JSP supported a motion of no confidence against the Liberal Democratic Party cabinet led by Kiichi Miyazawa.

Before the election, Yamahana agreed to have the JSP participate in a bloc with various opposition parties, masterminded by Ichiro Ozawa, to overthrow the LDP and implement political reform.

Although the JSP were still the largest party in the coalition, Yamahana was not considered for prime minister due to the dissatisfaction shown by the large loss of seats.

In June 1994 the JSP and the LDP formed a coalition with Yamahana's successor as party chairman, Tomiichi Murayama, as prime minister.

In early 1995, he aimed to formed a new party together with JSP Secretary General Wataru Kubo and others, but after the Great Hanshin earthquake the plans were scrapped as it was not a time to cause political turmoil.