In 1955, the two socialist parties reunited into the Japan Socialist Party, and Katsumata assumed the role of Diet Strategy Chairman. In 1958, when Wada was dismissed from his position as Policy Deliberation Chairman, Katsumata succeeded him in this capacity. Hereafter, Katsumata played a leading role in the Wada faction's activities, and distinguished himself in the Wada faction, which was primarily composed of ex-bureaucrats like Wada and Katsumada themselves, by trying to acquire greater political tact, something that was seen as lacking among ex-bureaucrats across the political spectrum. During the legislative debates surrounding the revised US-Japan Security Treaty, Katsumata joined the other socialist leaders in demanding clarifications from government figures over their contradicting comments concerning the nature of the treaty, attracting much public media attention. When issues relating to China–Japan relations came up in 1961, Katsumata was reported as being relatively less pro-Beijing than Kōzō Sasaki, and disputes over China were believed to be one of the main rifts within the JSP at the time.
