You'd be surprised how little difference there is between the parties. As someone once said: 'The Republicans and Democrats are like two empty beer bottles: they have different labels, but they're still nothing but empty bottles' So whichever one gets in isn't going to be that differen tfrom the other. Looking at the history, Americans tend to vote Democrat in times of poor economic performance. Then again, never in history has a black man been in the running for President with such a good chance of winning.
so really, it could go either way. I'd ignore the opinion polls in america for now: the candidates have only just started their campaigns, and the polls will probably swing more times than a 3 year old in a playground.