In March 17, 1947 in a conference in Rothenburg ob der Tauber Külz and Theodor Heuss were elected co-chairmen of the planned German Democratic Party aimed at uniting liberals of both Soviet and West occupation zones.
These plans never realized, though, as Wilhelm Külz, unlike East German CDU leader Jakob Kaiser, participated in SED-dominated German People's Congress for Unity and True Peace that took place in December 6, 1947. This brought about internal fights both within the LDPD as well as between East and West German partners of DPD. Although the LDPD leadership criticized that participation, it was unable to take any further steps demanded by the West German liberals.
On a session of the united leadership of DPD that took place in January 18, 1948 and which Külz refused to attend, Theodor Heuss argued that Liberal Democrats' unwillingness to take any measures against Külz proved their commitment to 'the Russian conception of German unity'. Upon in, Arthur Lieutenant, the spokesman of LDPD on the matter, declared that under those circumstances and considering reproaches laid against East German liberals, no further co-operation was possible. This was in fact the end of DPD.
Together with Otto Nuschke (CDU) and Wilhelm Pieck (SED), Wilhelm Külz led the German People's Council (Deutscher Volksrat), forerunner of Volkskammer of GDR. From 1945 on, Külz was the publisher of LDPD daily Der Morgen.
Wilhelm-Külz-Stiftung, a foundation close to FDP is named after him.