On the question of what impact TV has on viewers (especially as regards politics), John Zaller (1996) points out that in the US neither Democrat nor Republican voters realise their their respective campaigns largely cancel each other out, as opposed to either side ever actually winning in any grander sense.
I'm inclined to believe he may have a point. I suppose the only way we would know would be if one decided to drop the big bucks advertising campaign and see how they fared at the next election.
Zaller, J. (1996). The myth of massive media impact revived: New support for a discredited idea. Political persuasion and attitude change, 17.