I Led 3 Lives - a show about a guy pretending to be a Communist

Back when America was based

The series was loosely based on the life of Herbert Philbrick, a Boston advertising executive who infiltrated the U.S. Communist Party on behalf of the FBI in the 1940s and wrote a bestselling book on the topic, I Led Three Lives: Citizen, 'Communist', Counterspy (1952). The part of Philbrick was played by Richard Carlson. The "three lives" in the title are Philbrick's outward life as a white-collar worker, his secret life as a Communist agent, and his even more secret life as an FBI operative helping to foil Communist plots.
https://en.wikipedia.org/wiki/I_Led_3_Lives