Things happening in life, even President Reagan almost left离世 by a gun shot, before his term's ending, while, President Clinton almost had been impeached.
The reality is that, the chance for A Vice President(法定接班人) taking office in this country, is quite high. Then, it becomes the chance we as voters have to take (and a decision which has to be made) right now before hand.
Senate McCain's health records, had been said, were not as strong. I would think mainly perhaps due to his 5 years' POW experience in Vietnam, which could set his life path that much off his family Healthy Inheritance (as you have mentioned from his mother side). On the other hand, if we changed our life style, we can surpass our family inherited life span.
Back to the race, Obama, as an uprising political character/figure in this country & to the world, does face enormous challenges of his own and the time of history, like the article mentioned racism as the number one, of course, age -- another, being suspected as Muslin, his wife's growing-up background, etc.