According to clinical psychologist Drew Westen, swing voters make choices based on emotion, not reason. 

According to Drew Westen, clinical psychologist and author of “The Political Brain: The Role of Emotion in Deciding the Fate of the Nation”, swing voters actually base their decisions on the following, in this order:

1.Feelings about the Party – what we should be focusing on.
2.Feelings about the candidate
3.Feelings about the issues
4.Rational evaluation of the Party
5.Rational evaluation of the candidate
6.Rational evaluation of the issues – what our current poll-driven strategies are based on.

According to Westen, they all make up their minds before they get to #4!