On the radical Left, Upton Sinclair is mainly known for being a former Socialist candidate for Governor of California, and for writing The Jungle, his 1906 novel about the practices of the meat-packing industry (the nationwide disgust produced entire regulatory bodies to enforce food safety standards within a matter of months).

He was also super into the paranormal. This letter vouches for a psychic who won over Sinclair and his wife:

McDougall, my wife and I had a very interesting experience with him: I had four envelopes, tightly sealed and absolutely opaque. Each contained a letter form a person no longer living. Ford started talking about Jack London and told a lot about him, and then said, “You have a letter from him with you.” The four letters were in an inside pocket, and not visible. I gave him all four of the letters, and he kept turning them over in his hands in broad daylight while we watched him. They remained unopened to the end. He told us several things that were correct, about the contents of three of the letters. I made notes of what he said, and afterwards McDougall, my wife and I checked over the notes, and McDougall agreed with us that there was undoubtedly supernormal power

Call me skeptic, but seeing as this letter was written in 1938, and it was well known that Jack London and Upton Sinclair were both friends and socialist comrades, it feels a little like a lucky guess to me.

Sinclair’s 1930 book Mental Radio included anecdotes of his wife Mary’s telepathic experiences. William McDougall (referred to in the letter above) wrote an introduction to it and later established the parapsychology department at Duke University.
 

 
Via Bolerium Books