Since 1945, Arndt was a member of the
SPD. In the 1950s, he was a member of the management and participated in creating the
Godesberg Program. In 1948 and 1949, he was a member of the economic council of the
Bizone, chairing the board for law, clerk law and "DM-Eröffnungsbilanz" (statement of the financial conditions for the
Deutsche Mark). From 1949 to 1969, Arndt was a member of the
Bundestag for the
SPD. From 1949–61, he served as corporate attorney and secretary of the SPD
faction. In addition to that, he served in 1951/52 as deputy managing director of the Bundestag board for Law and Constitutional law as well as deputy managing director of the Bundestag board for reviewing the federal administration (the so-called
Platow-Ausschuss). From 1953 to 1961, he chaired the research group "Rechtswesen" of the SPD faction. Arndt is most famous for his speech in the debate about statute-barred prosecution of Nazi crimes, in which he states that morally feels guilty about taking part in those crimes. In many cases, Arndt represented the SPD faction before the
Bundesverfassungsgericht. From 11 March 1963 to 31 March 1964 Arndt served as senator for science and arts in
Berlin. == Writings ==