Obama will probably win for the following reasons-
1) As previously stated, the Republicans are truly awful. Romney is spending 9 figures to beat three guys who were carrying their own luggage back in December.
2) 69 million people voted for Obama last time. Human nature is such that people don't easily admit they made a mistake. It takes a very convincing argument to get people to admit they goofed.
3) It is rare for the electorate to throw a party out after only four years in the presidency. We've voted out incumbants, to be sure, but usually after their party had been in charge for a while. The last time it happened was 1980, when we threw out Carter after 4 years. Before that, the last time was Benjamin Harrison. And it was questionable if he even won a first term legitimately.
4) Monetary advantage. - Obama will have a lot of money. Romney really won't. Unless he taps into his personal fortune.