Fortunately, there's no good evidence that subliminal messages have any real effect:
http://www.rapidnet.com/~jbeard/bdm/...ogy/sublim.htm
I am aware of some studies that show some effects on task performance using subliminal messages of a Freudian nature (eg. "It is OK to beat Dad" improves performance, "It is wrong to beat Dad" worsens performance), but on the whole, the concept of subliminal messages is unproven and based mainly in a fraudulent "study" done in the 50's.
Nonetheless, I wouldn't put it past American politicians to try such dirty tactics anyway.