Cranston was a good guess :-) He had a longer history with the antiwar movement than Tunney and brushed shoulders with CPUSA pretty often, but I haven't been able to document him actually working for the KGB. He and Tunney did work closely with the VVAW in the early 70s, though.
Time will tell on Cranston.
Even as a child, I knew about that piece of work and I despised him.