Political scientists love to discuss this topic. The short answer is simple, though: If they vote for my guy, they're rational - and vice-versa. Political scientists seem to think that voters should be more rational.
Of course, neither voting nor politics are entirely rational, nor should they be. Life is too complex for logic, and non-rational doesn't mean stupid.
The subject is discussed in Irrational Electorate by Larry Bartels, in Wilson Quarterly.