When guys like Romney and Gingrich are getting attacked for not being conservative enough, I don't see how the GOP has a chance no matter who they are facing. They have a weak field and are divided.
And Obama is going to be tough to beat. He knows the game well, will have huge resources available, and seems very organized and focused.
Additionally, Obama has been positioning himself more towards the center of late, and for the most part avoiding getting dirty (such as in the budget debate).
The loudest part of the GOP seems intent on being far right, which may keep a moderate candidate from winning the nomination. Politically, it is tough for a far right candidate to beat a centrist. I just don't see it happening. I'm not saying Obama is a centrist, but he can play one for awhile, which politically is pretty smart.