“Jill Stein is a useful idiot for Russia. After parroting Kremlin talking points and being propped up by bad actors in 2016 she’s at it again,” DNC spokesman Matt Corridoni said in a statement to The Bulwark. “Jill Stein won’t become president, but her spoiler candidacy—that both the GOP and Putin have previously shown interest in—can help decide who wins. A vote for Stein is a vote for Trump.”
In this instance, “useful idiot” has a very specific meaning:
https://www.merriam-webster.com/dictionary/useful idiot
: a naive or credulous person who can be manipulated or exploited to advance a cause or political agenda
It is one task of the KGB [in 1982] to apply its skills of secrecy and deception to projecting the Soviet party’s influence. This it does through contacts with legal Communist Parties abroad, with groups sympathetic to Soviet goals, with do-gooders of the type that Lenin once described as “useful idiots” … .
Re: the Lenin thing:
https://en.m.wikipedia.org/wiki/Useful_idiot
“A useful idiot or useful fool is a pejorative description of a person, suggesting that the person thinks they are fighting for a cause without fully comprehending the consequences of their actions, and who does not realize they are being cynically manipulated by the cause’s leaders or by other political players.[1][2] The term was often used during the Cold War to describe non-communists regarded as susceptible to communist propaganda and psychological manipulation.[1] A number of authors attribute this phrase to Vladimir Lenin, but this attribution is not supported by any evidence. Similar terms exist in other languages.”