People have been sending tea bags to DC for many years. The present day use of the word “teabaggers” is purely derogatory. You should read the source link...there is a link that discusses MSNBC’s extensive use of it, complete with the double entendres.
As I said, it’s time to “appropriate” the term and turn it back on them. Take ownership of it (in political circles and in “language”) and take the ground out from under them ... that’s how you can combat it ... :-)