I might normally be a right-winger but in the case of the US it's different.
They needed a fresh start as far as the Presidency went, and ideally a line against the former racist stance of the country (although I have often seen it suggested that the US needs a Native President, not a black President, to wipe that out for good), but - as SS4 states above - the Republican majority have made it difficult for Obama to do his job. It's not so much "No we can't" as it is "No I'm not permitted to". The Health Bill was a definite plus - will we get such initiative from the Republicans? Somehow, I doubt it.
Obama winning was good for everyone - not just the US, but the world. If he loses and Romney isn't a massive improvement on Obama - whose struggles have largely been due to the opposition, including some related to the idiot['s junior?] before him - then it will show what a farce the US system is. Here, FPTP works*. Over there, I'm not sure, given the relatively low number of swing states (most of which are lesser states from the electorate perspective anyway) - there are more red states than blue states, but the majority of the higher-value coastal states are blue.
The only other thing I would say is that if Obama loses, will the US elect a non-whtie candidate again? If any other non-white candidates should lose, will the age-old racial criticisms of the US resurface?
In short, if I was an American, I would vote Obama and thus Democrat. Not so much because I think he's done a good job, or even because he's black, but more because he has a lot of potential as he showed early on before the Republicans on Capitol Hill strangled him.
*Minimal apologies offered with a somewhat sarcastic tone to the pro-AV lot on here.