When was the last time that the nice guy lost the Presidential election? I know it's hard to pin this down, but really I think you'd have to go back to Nixon to find a winning candidate who was not the kind of guy who you could imagine as just being "nice".
Will the 2008 election see the end of the nice guy's run? If the election comes down to Rudy Giuliani vs Hillary Clinton I think the answer is most definitely yes. Neither of those two could be considered 'nice', although I think Hillary has done an awful lot to try and soften her image, to adopt some of Bill's charm.
Maybe Giuliani will do the same, but I just can't imagine him coming across as pleasant, charming, nice. He's mean and he knows it. So does the American public or at least those who know much about Giuliani. There's a long way to go, but if Giuliani keeps rising in the polls I will conclude that Americans have decided to agree with the unlikeable baseball player/manager Leo Durocher, who famously said that "nice guys finish last".