Red Dupes would also work but "Useful Idiots" does better to identify the aid and comfort that they provide Americas (non-communist) enemies.
The left is getting outraged that protestors are being labeled communists but they are doing nothing to discredit the level of communist backing in the antiwar movement.
They've been had and used as pawns in a larger political game and they aren't happy about it but as always, the left cannot admit that they were wrong.