The word ferment means a commotion or excitement. Consider that the fermentation that turns juice or grain into alcohol is the result of the agitated development of bacteria, and then you can better appreciate the word's meaning.
While you might be more familiar with the verb ferment, referring to the natural process that produces alcohol, the word is also used as a noun, giving the same type of boiling emotion to people or events. If you keep in mind that the root of the word is fevere, meaning "to boil or seethe," you get a vivid picture of the ferment experienced at a political rally where tempers run high.