I think that urban myth was debunked 30 years ago.
I think, it's a mixed bag.
Quoting from EXPLORING PSYCHOLOGY 7TH ED BY DAVID G MYERS p141:
"Can we be affected by stimuli so weak as to be unnoticed? Under certain conditions, the answer is yes. An invisible image or word can briefly prime your response to a later question. In a typical experiment, the image or word is quickly flashed, then replaced by a masking stimulus that interrupts the brain's processing before conscious perception. For example, one experiment subliminally flashed either emotionally positive scenes (kittens, a romantic couple) or negative scenes (a werewolf, a dead body) an instant before participants viewed slides of people (Krosnick & others, 1992). Although consciously perceiving either scene as only a flash of light, the participants gave more positive ratings to people paired with positive images. People somehow looked nicer if their photo immediately followed unperceived kittens rather than an unperceived werewolf."
"This experiment illustrates an intriguing phenomenon: Sometimes we feel what we do not know and cannot describe. An imperceptibly brief stimulus often triggers a weak response that can be detected by brain scann ing (Blankenburg & others, 2003). That small brain response may evoke a feeling, though not a conscious awareness of the stimulus. The conclusion (turn up the volume here): Much of our information processing occurs automatically, out of sight, off the radar screen of our conscious mind."
"But does the fact of subliminal sensation verify entrepreneurial claims of subliminal persuasion? Can advertisers really manipulate us with "hidden persuasion"? The near-consensus [Qx emphasis] among researchers is no. Their verdict is similar to that of astronomers who say of astrologers, yes, they are right that stars and planets are out there, but no, the celestial bodies don't directly affect us. The laboratory research reveals a subtle, fleeting effect. Priming thirsty people with the subliminal word thirst might therefore, for a brief interval, make a thirst-quenching beverage ad more persuasive (Strahan & others, 2002). But the subliminal tape hucksters claim somthing different: a powerful, enduring effect on behavior."
When Anthony Greenwald and his colleagues (1991, 1992) tested that claim in 16 experiments evaluating subliminal self-help tapes, his results were uniform: None had any therapeutic effect beyond that of placebo (the effect of one's belief in them). His conclusion: "Subliminal procedures offer little or nothing of value to the marketing practitioner" (Pratkanis & Greenwald, 1998)."
I believe the text so far as it goes.
HOWEVER, I have a couple of caveats in mind . . .
1. Dr Milton Erickson of Phoenix was a master at curing hopeless cases a long string of other excellent therapists had failed with. A major aspect of Milton's methodology was subtle hypnotic cadence, words, tones, phrases etc. which the client would have no conscious awareness of in terms of as a feature of the treatment program. Yet, they had very enduring and powerful effects.
2. Bandler and Grinder studied Milton and some other best therapists and derived NEURO LINGUISTIC PROGRAMMING. OThuga used those techniques to great destructive effect on our Republic in the last election. Most watching his NLP laden speeches had no idea he was manipulating them unconsciously. Yet, he did, very effectively.
3. Various well known 'agencies' imho, are undoubtedly using such methods wholesale on the populace as well as on hapless prisoners they want information from . . . as well as on others they have cocked and primed to do dirty deeds. So I believe the studies cited above . . . I don't believe that 100% of my profession is being 100% honest in all they are reporting on publicly. Or, perhaps some studies are just never reported on by the researchers.
However, let's just take the studies above . . . PRIMING THE PUMP to trigger an EMOTIONAL response at a key point in a key direction by masses of people . . . could easily have long lasting effects at the ballot box. It would likely take maybe dozens if not 100's of repetitions followed-up by very sophisticated brainwashing sequences . . . but I think we saw in the last election how it could be very powerful in terms of very long lasting impacts on a society.
My $2.00 worth. Perhaps Petronski will enjoy trashing the above maybe just out of reflex or on unprinciple or for some other mystifying "reason."