In 1917 following the split within the SPD, he joined the left-wing faction which formed the Independent Social Democratic Party (USPD).
In April 1919 in Munich, he took the position of People's Delegate for the Interior (Volksbeauftragter für das Innere) in the short-lived Bavarian Soviet Republic.
He additionally served as a city councillor and later mayor in Schweinfurt, switching his party affiliation in 1922 back to the SPD.
After the Nazi seizure of power, as a prominent social democrat Soldmann found himself subject to persecution by the new regime.
Although Soldmann's parliamentary mandate had been reconfirmed during the 1933 election, in the same month he was placed into protective custody (Schutzhaft) despite his immunity as a parliamentarian.
He was therefore unable to participate in the vote on the Enabling Act, which formed the legal basis for the establishment of the Nazi dictatorship.
After the outbreak of World War II, he was arrested again in September 1939 and imprisoned in Sachsenhausen concentration camp until the spring of 1941.