[dash2]

It was a slur against collaborators with communism, not against social democrats: https://quoteinvestigator.com/2019/08/22/useful-idiot/

[DiogenesKynikos]

The slur was originally directed against social democrats by conservatives who accused them of helping communism.

Later, the term became a much more general slur used by conservatives against anyone who wasn't sufficiently "tough" against the USSR.