In my experience, those that vote "for" someone typically just do it because they've always voted that way, they're too stupid or set in there ways to change, they don't want to actually think and evaluate the candidates/parties, they think all of them are the same and so vote for those closest to their point of view regardless of how well they'd do or have done. Those that actually think, pretend to, or just throw their hands up in disgust typically vote "against" someone.
As an aside. I saw a financial chart once in a bank showing the Canadian and US governments for the last 30 or so years and the recessions for each country. It was a real eye opener and made me wonder why anyone would ever vote republican.