With the establishment of the Westdeutschen Verlag [de] at Cologne and Opladen in 1946, Middelhauve also founded one of Germany's leading publishing houses for works of the social sciences.

However, by January 1946, he became affiliated with the newly-established Free Democratic Party (FDP) in the British occupation zone and became an associate of Franz Blücher, Hermann Hopker-Aschoff and Erich Mende, notable leadership figures in the Rhineland.

While state party chairman, together with fellow Landtag member Ernst Achenbach, he formulated the nationalist "German Program", which advocated for a united Germany and against the "victors' justice" of the Allies.

Middelhauve, through his personal secretary Wolfgang Diewerge, also had contacts with the Naumann Circle, a clandestine group of Neo-Nazi conspirators that was planning to infiltrate the FDP as a means to return to power.

[1] An FDP internal investigation resulted in the dismissal of Diewerge and two other minor officials but Middelhauve was able to retain his state and federal party posts.

[6] As the chairman of the FDP state parliamentary faction from 1946 to 1954, Middelhauve was the leader of the opposition to the CDU–SPD–Center Party coalition cabinet of Minister-president Karl Arnold.

When the FDP joined the CDU and Center in Arnold's next coalition cabinet in July 1954, Middelhauve became Deputy Minister-president and Minister of Economy and Transport.

[4] Middelhauve, who had backed Arnold, was not included in the new cabinet, and within days he also resigned as FDP state chairman, two years before his term was to expire.