useful idiot

noun2

1. •
   (disapproving) One who unwittingly supports a malignant cause through naive attempts to be a force for good.

   • But what should the Obama Cult do now? The man has turned off the switch on the "Hopenosis" and has revealed the "Change Brigade" for the useful idiots they were. — 2009
2. •
   (disapproving, historical) A Communist sympathizer in Western countries, from the perspective of the political right.

   • L'Umanita said the Communists would give the “useful idiots” of the left-wing Socialist party the choice of merging with the Communist party or getting out. — 1948