For many people is very hard to understand the mechanics involved in influencing public opinion. And unfortunately, I see this mainly as a problem of liberals, and members of the progressive movement. I think this fact accounts for one of the most significant weaknesses of the progressive movement.
When I debate with an intellectual liberal I notice that there is tendency to rely on facts, and data, and empiricism (who would've thought!). The liberal progressive is very uncomfortable dealing with an interlocutor who is espousing utter nonsense, stupidity, and dogma. When she interact with such individual she tends to try to explain, to inform, to educate, and when she is unable to, then she gives up, moving on with full knowledge that the right-wing nutjob she's trying to influence is beyond help.
And herein lies one of the biggest weaknesses of the Left: The belief that reason, and common sense, and science, and empiricism are the most important components when it comes to shaping public opinion. That attitude is extremely dangerous, and will always be a major contributor to the defeat of liberal and progressive causes.
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http://www.dailykos.com/story/2009/12/10/812766/-A-Very-Dangerous-And-Radical-ManifestoPart-III