Euler was one of the co-founders of the
Liberal Democratic Party, later the FDP, in
Kassel in 1945 and then throughout
Hesse on 29 December 1945. In 1946, he initially became state manager, and in June 1947, he became state chairman of the
FDP as the successor to
Georg Ludwig Fertsch. He held the office until he left the party in 1956. At the founding party conference of the FDP in
Heppenheim, Euler was elected to the executive board of the federal party. At the 1949 federal party conference of the FDP in Bremen, he wanted to enforce a commitment to rearmament but failed due to the anti-militarist sentiment of the majority of delegates. At the meeting of the Federal Main Committee of the FDP on 21 September 1950, he spoke out in favor of resolving not only the
incompatibility of membership in the FDP with membership in the Association of those Persecuted by the
Nazi Regime but also with membership in the
German Peace Society (DFG). While the first demand was passed with a clear majority because of the KPD's decisive influence on the
VVN, the proposal regarding the peace society failed; after all, , an FDP member, was the federal chairman of the DFG. At the federal party conference in September 1951 in Munich, he ran against the previous deputy federal chairman,
Hermann Schäfer, and was narrowly defeated by 114 votes to 139. Together with
Hans-Joachim von Merkatz, Euler advocated a rigorous end to denazification in 1950. Both sought to exonerate the so-called main culprits and those accused of all current or impending sanctions at the time. Euler was one of the strictest representatives of the citizen bloc orientation of the FDP. In 1952, he called for the
Baden-Württemberg FDP/DVP, which he described as demi-Marxists, to be excluded from the party after
Reinhold Maier entered into a coalition with the
SPD in the southwestern state. The FDP's coalition change in
North Rhine-Westphalia in 1956 from the CDU to the SPD ultimately led to a group of FDP members and MPs under Euler's leadership breaking away from the party in February 1956 and founding the
Free People's Party (FVP). With the FVP, Euler came to the
German Party (DP) in March 1957. == MP ==