Sage advice from Sean Quinn at Nate Silver's site Five Thirty Eight:

"Because they already do. That ship has sailed. When facts are used to
discredit Sarah Palin, emotion trumps facts. The instinct is to defend
against the facts. Consider: you meet someone and like him or her on a
gut level. A stranger – someone who doesn’t have built-up personal
credibility with you – gives you a list of reasons not to like that
person. How do you react?

On an emotional level, you want them
to be wrong, and you will take every possible favorable inference on
the likable person’s behalf. Using facts is pushing a big rock uphill.
You might get it to the top with a few voters, but you’re going to
expend a lot of energy for only a little return."

http://www.fivethirtyeight.com/2008/09/dems-must-give-voters-explicit.html

Read the whole thing for how to counter this, it applies to McCain too.