Never get into an argument with people about their emotional reactions. If someone says they are supporting McCain/Palin because they are impressed by their life story, etc. then they are expressing an emotional reaction. If you try to counter that with a purely factual argument you will lose because emotions aren't based on facts and always trump facts.
Here's how I might approach it:
I respect that you were persuaded by McCain's compelling life story. I am impressed by it as well. But here's the thing that bothers me: To many con men
throughout the ages know that they quickest way to win over a mark is
to appeal to their emotions. So, when I see a politician appealing to
my emotions, a part of me automatically ask, "What are they trying to
sell me?" I look past the emotional imagery and ask myself, "What will
a President McCain really mean for me?" And when I do that I find that
<insert a list of 2-3 facts that argue against McCain>.
Not saying this will work. It probably won't. But at least you won't
come off as belittling of their emotional connection to McCain.
Put it this way, you may have a strong emotional connection to many
people in your family, but that doesn't mean you would trust them to
run the country.