In Hesse, the process of democratization began in early 1946. A key aspect of this new democratic beginning in Greater Hesse, which was proclaimed on September 19, 1945, was the drafting of constitutions in the individual states of the American occupation zone.

On February 4, 1946, just a few days after the first free elections at municipal level, the headquarters of the US military government OMGUS issued its directive on the preparation of state constitutions. Following this directive from US General Lucius D. Clay, who resided in the IG Farben building in Frankfurt am Main. Clay, Prime Minister Karl Geiler, himself appointed by the military government, appointed the Preparatory Constitutional Committee.

This committee, which was still pre-parliamentary, consisted of 13 people, including members of the cabinet such as Minister Werner Hilpert (CDU) and Georg August Zinn (SPD). It was chaired by Minister President Karl Geiler. When it began its work on March 12, 1946, this committee of experts initially drew up a draft law for the election of the Constitutional Advisory Assembly. According to this electoral law, the Constitutional Advisory Assembly was to consist of 90 members, who were to be elected on the basis of proportional representation and majority voting. Between April and June 1946, the Preparatory Constitutional Committee drafted the constitution for Greater Hesse. This draft constitution bore the signature of the Heidelberg constitutional law expert Prof. Walter Jellinek, who successfully advocated a central catalog of inalienable fundamental rights.

With the first state-wide election to the constitutional advisory state assembly on June 30, 1946, the constitution-making process entered a parliamentary stage. Out of 90 seats, the SPD won 42 seats, the CDU 35, the KPD 7 and the LDP 6. The constituent session of this parliament took place on July 15, 1946 in the auditorium of the Realgymnasium in Oranienstraße, today's Oranienschule.

In the further course of the constitutional deliberations, the SPD resisted the political temptation to push through its ideas of a "socialist democracy" together with the KPD. Instead, after difficult negotiations, the SPD and CDU finally reached the legendary Hessian constitutional compromise on a broad political basis, which was to prove to be a long-term foundation. Following the approval of the draft constitution by the US military government on October 29, 1946, the referendum and simultaneous election to the Hessian state parliament took place just over a month later. On December 1, 1946, 76.8% of voters approved this first German state constitution after the Nazi dictatorship.

In the state parliament elections for the first legislative period, the SPD was again able to achieve a victory. In the Wiesbaden City Palace, the seat of the Hessian State Parliament, it continued the cooperation from the constitution with the second-placed CDU in a grand coalition.