First off I want to get into what politcal propaganda is all about. Propaganda is a form of communication such as writings, audio, or posters, that are aimed at influencing the attitude of a community toward some cause or position. Most propaganda is usually biased and negative towards a certain cause, but other types can be neutral. You can see most of it today in the upcoming Presidential election with all the runners making T.V. commercials.(SOURCE) http://en.wikipedia.org/wiki/Propaganda
I found this video online, it is slow but the speaker makes some really good points about why and how political propaganda works.
http://youtu.be/djKKFYCNeoA
Propaganda is so powerful during times of war. Propaganda has been used to promote war as well as promote peace. During both world wars there were many posters that were used to evoke an emotion from inside. In the Cold War times both countries used propaganda extensively. There was propaganda on the tv, on the radio, and even in movies. (SOURCE) http://www.sourcewatch.org/index.php?title=Propaganda
Posters are probably some of the most effective propaganda tools. There have been so many made and some are extremely powerful.
You know the saying jump on the bandwagon? It's an actual term used for propaganda with political parties. During the campaining process, this is used to build momentum and followers. It is used to pressure and persuade people with common interests to vote for that certain person.