My guess was always that this mole was Ron Dellums. His communist sympathies weren't exactly secret, but that didn't keep him off the Armed Services and Intelligence committees. Hasn't hurt him politically, either.
Dellums was another one. But his involvement with CPUSA was pretty blatant and not much of a secret to the intelligence community, I think. The year he was elected he went to a World Peace Council function which was monitored by US intelligence and was noted in a Congressional report on the antiwar movement's CP/SWP ties c. 1970, IIRC.
Harold Ickes the Elder and Junior.