A "useful idiot" in politics is a pejorative term for someone who supports a cause or group, believing they're advancing good intentions, but without fully realizing they're being manipulated or exploited by its leaders to further an agenda with broader or unintended consequences. The phrase gained prominence in the mid-20th century (often tied to Cold War discussions of unwitting allies to communist movements), though its exact origin isn't Lenin as sometimes claimed—earlier uses appear in the 1800s for naive political dupes.