I think that you can look at the political ads that are produced and aired by the candidates to help answer this question. Most of the ads are attacks on other candidates with very little focus on the issues.
Ever since television became a major force in American socities, presidential candidates have been using the media to center the campaigns on their personalities and images as opposed to centering campaigns on things like issues or party identification. By doing so, they have created a system where we tend to elect our presidents based largely on those personal qualities than on their political positions as such.
Presidential candidates do this mainly through their use of commercials. They produce commercials that are meant to give them a particular image rather than to promote their political opinions. Candidates also do this by appearing in carefully stage managed settings that are meant to project a particular image. For example, candidates like to do things like going hunting to show that they are in touch with the common people.
By doing things like this, the candidates create campaigns that are about them -- about their personalities and their images.