The main reason is that his candidature will racialise American politics like never before (well, at least, since Wallace stirred things up in the late sixties). For too long, the media myth has been America as a melting pot (I suppose Obama is a living example of that 'myth', but remember his father was an African student and his step-father Indonesian, rather than an Afro-American) when the reality is quite different. Obama's candidature will rub some noses in it (not least because of his dubious connections with corrupt businessmen, radical preachers and sixties underground 'class warriors'). I expect some chickens to come home to roost in American politics over the summer. Go Obama, go!