Why do politicians and more likely, their ""advisors" think that negative advertisements are the only ways to win an election?

I just saw or got bombarded with the same nasty, vicious and offensive McCain advertisement that liks Obama to Bill Ayers. a terrrorist and then segues into blaming the entire sub-prime mess on Congressional “liberals”. Do these guys think that juxtaposing terrorist and liberal they are going to garner votes for McCain? But there are also so many ads that only contain nasty slurs and nothing positive about their own candidate, Democrats., Republicans and the candidates who refuse to state what party they belong to. My wife and I are just sick of the whole thing. Elections should be decided by issues or competency and not by what happened in 1969 and not by whose daughter became pregnant at the age of 17.