I remembered this term from taking Russian History in 1978. In researching the term I found (and remember ) that the term was first used by Joseph Stalin and Vladimir Lenin. It was the term Lenin and Stalin used in referring to the liberals in the United States in that era. The reason for this was because their perception was that the liberals in the 1920's and 1930's were easily manipulated and naive. They could be easily bent to advance the Soviet cause and were a highly expendable asset.
Who are the useful idiots today? I invite you to a vigorous and civil debate about current events....