etymological lesson...

preaching to the choir:

"Preaching to the choir", "preaching to the chorus", and "preaching to the converted" all mean the same thing, that the people you are trying to convince already believe in what you are saying. Example: "I agree with you entirely; you are preaching to the choir." You are "preaching to the choir" when you talk (preach) to people to convince them of something they already believe. Example: "Don't just stand there preaching to the choir; get out and find new members!" The members of the "choir" play the music, while those in the "chorus" sing along, as the preacher preaches to change (convert) the non-believers into believers ("the converted"). People tell you that you are "preaching to the converted" to let you know they support your idea; they also might be saying you don't need so spend so much energy to make them believe what they already believe. Example: "You don't need to tell me this project is important; you're preaching to the converted."
GoEnglish.com Idioms

Both the radical right christianists and the progressive left are guilty of the same thing. They continually preach to the choir when they think that they are trying to gather new supporters or change people's minds toward their ways of thought.

I'm not sure it really works that way. The people who are really committed to one side or the other are going to be committed no matter what. The two sides are preaching to the choir because the majority, those who fall someplace between the two, are actually listening to a different tune. Or, maybe the choir isn't listening to the correct tune.

think about it...