After Nishio saw the writing on the wall and bolted the JSP in January 1960 to form the new Democratic Socialist Party, Eda rose to become the second most powerful man in the JSP, serving as Secretary General under new party Chairman Inejirō Asanuma. When Asanuma was spectacularly assassinated during a televised election debate on October 12, 1960, just weeks before a national election, Eda was hastily named "Acting Chairman" of the JSP and became the party's leader and candidate to become prime minister of Japan should the party triumph in the election.