The way I see it is, Obama is an outlet for all the americans who hate bush and want change, which unsurprisingly just so happens to also be the basis for his campaign slogan.
McCain on the other hand is an outlet for all the 'conservative' white folk who are set in their ways. Those ways being xenophobia if not outright old fashioned racism. A lot of his supporters, support him more out of hate for Obama than agreement with his views.
If you've seen some of McCain's latest town hall rallies, the nut jobs amongst his supporters have finally surfaced, as they shouted and aired a number of their highly offensive opinions about Obama. They even booed McCain when he denounced (as he has done numerous times during his campaign) the crazier views his campaign and supporters have seen fit to spread so eagerly. This raises the question I put forth earlier about whether they are supporting him or whether they merely hate Obama.
Of course, I do not believe that all of McCain's supporters are nut job american racists, nor that Obama is only supported due to Bush's low approval ratings. Still, it is interesting to view the different campaign strategies which they have employed, as both are aimed at specific groups within the USA. Obama has been all about change and tying McCain to Bush, while McCain's has been all about attacking(if not smearing) Obama's character.