I agree with Kings' second point here and will expand upon it a bit as follows:
If you come across as some sort of redneck lout it will have a negative effect on the recipient. Don't make ill advised comments about the opposition,don't abase them and for goodness sake don't even think about using any swear words or slang in your correspondence.
If you want to post a proposed letter here before sending it I'm sure you'll get excellent help in wordsmithing.
I will be preparing a new letter to send to my congressional representative who is a moderate Democrat and who I hope to swing over to our way of thinking. Fortunately I reside in a State where being anti-gun is usually political suicide so I'm optimistic in this case.
Jim