The title pretty much says it all. I thought it would be easy to find the origin by googling, but although there are many references, none of the ones I scanned included the origin.
I remember first seeing "congresscritter" decades ago, probably in Analog, maybe in an editorial by Stanley Schmidt. But even if my memory is correct, did Schmidt create the word, or just use it?
I also remember that I first saw congresscritter when words like chairman, congressman and mankind were being attacked as sexist. So that suggests early to mid 1970s for the origin.