The issue with subliminal messages, whether used in commercial ads, in political campaigns, or anywhere else, is that you are attempting to deceive or manipulate the person (or people) viewing the message by inserting it beneath the level of his/her conscious awareness. When you make a suggestion that a person is aware of, you allow this person to assess whether or not it makes sense to him/her. When you bypass the recipients conscious mind, you are depriving him/her of the chance to consider the merits of your message, and make more likely your ability to manipulate his/her emotions. This raises obvious ethical and legal issues. However, there is apparently no solid proof that subliminal messages can force a person into doing something that departs significantly from behavior they would have followed without the subliminal message.

According to http://www.umich.edu/~onebook/pages/frames/legalF.html :

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Are subliminal messages legal?

In Australia and Britain, the use of subliminal advertising has been banned with severe consequences for those who disobey the strict laws. In the United States, not much is being done from a legal standpoint to curn the use of subliminal messages. The Federal Communications Commission, however, will now revoke a company's broadcast license if the use of "subliminal techniques" is proven. The following is from the FCC's Manual for Broadcasters:

"We sometimes receive complaints regarding the alleged use of subliminal techniques in radio and TV programming. Subliminal programming is designed to be perceived on a subconscious level only. Regardless of whether it is effective, the use of subliminal perception is inconsistent with a station's obligation to serve the public interest because the broadcast is intended to be deceptive." (Federal Communications Commission Record, 2001)
...
Judas Priest is a British heavy metal rock band-one of the first bands of the genre. Their popularity peaked in the mid-seventies, and in 1978 they produced an album called Stained Glass. It is because of that album, and an alleged subliminal phrase hidden in the song "Better by You, Better than Me," that the band had to go through extensive trial proceedings that lasted over a year.

The hidden phrase was, apparently, "do it." In isolation, this phrase has little meaning unless there is some antecedent to which the "it" refers. But, according to the parents of the two teenage Judas Priest fans who attempted suicide in 1985, a hidden "do it" can have much more serious implications.

In Reno, Nevada, in the summer of 1989, the boys' parents took Judas Priest to trial, suing the band for the influence that their music allegedly had on the boys' actions. The parents argued that their sons, who probably already had suicidal tendencies, were influenced enough to take action after experiencing the message in Judas Priest's music. Judas Priest claimed that they did not intentionally place a subliminal message on the album, and made the argument that, even if they had used subliminal messages, the messages should be protected by the First Amendment.

The judge, Justice Jerry Carr Whitehead, ruled that the First Amendment's protection of freedom of speech and press does not extend as far as subliminal messages. Since the recipient of a subliminal message is unaware of it, the message can't contribute to dialogue, the pursuit of truth, the marketplace of ideas, or personal autonomy. There is no information exchange when it comes to subliminal messages, and no disagreement or argument is possible if recipients are unaware of the message's presence. Judge Whitehead also explained that people have a right to be free from unwanted speech. Since subliminal material cannot be avoided, it constitutes an invasion of privacy (Vance v. Judas Priest 1989b).

Justice Whitehead ruled, however, in favor of Judas Priest. His ruling was based on the defense's insistence that the power of such a message to move a person to action has never been proven (for more on this idea, look at the psychology section of this website). He stated his conclusions on the subliminal threat in this way:

"The scientific research presented does not establish that subliminal stimuli, even if perceived, may precipitate conduct of this magnitude...The strongest evidence presented at the trial showed no behavioral effects other than anxiety, distress or tension." (Vance v. Judas Priest, 1990)
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Some related reading:

- http://www.lawpublish.com/subliminal.html