Answer: propaganda is based on supposition of malleability of human nature. In late 19th century there were many theories (psychology of masses, i.e.Gustav Le Bon or Italian sociology of elites, i.e. Vilfredo Pareto, Mosca etc etc.) which propounded idea of malleabilty of human nature and human psyche. The same can be said about psychoanalysis. The result of that was use of propaganda in 1930s and 1940s (press, radio), establishmemt of ministries of propaganda etc etc. The impact was only short-lived because the intention was to monopolize public space, immunize some social theories and produce closed society.
Explanation: propaganda contradicts idea of democracy. Democracy requires plurality of thoughts, ideas, patterns, behaviour, attitudes, free circulation of all that. Democracy requires open society.