I believe that Republicans vote Republican and Democrats vote Democrat....the campaign managers know this and they have a pretty good idea of how many of each demographic they have to work with. So, their job isn't to try and convince you who to vote for, you've already decided that. Their job is to get you into the polls so you can actually cast that vote.

Somewhere along the line, they decided that the best motivator was to get you pissed off enough at the other guy that you would make the time to get into the polls.

I think this is basically true. Most people who care about politics are too brainwashed by punditry to take an objective position on various candidates for office. Most people who don't care about politics are too overwhelmed by the sheer number of different little offices and races they are asked to vote for, and the dearth of good information about the candidates running for them, that voting a party line is just easier. Then there is social peer pressure. "You're not a pussy liberal are you boy?" or "God! You're not a Republican, are you?! Yuck!" No one likes an independent. It means you're everyone's enemy.