I think it all comes down to how Obama will work with the Republicans. If he pulls a Bill Clinton and knows when to work with them and when to fight them (the economy would also have to improve), he will win. However, Obama so far has shown that he doesn't know how to pick his battles and on the ones he does pick, he does a really bad job at fighting (fighting Fox News, spending all of his political capitol on health care reform, etc.).
It also will come down to who the Republicans nominate. If a Tea Party candidate is elected, they will have to be more like Marco Rubio than Sharon Angle, Ken Buck, Sarah Palin, and Christine O'Donnell. The nomination is going to have to come down to who will be a competent candidate, have a personality that doesn't look like a crazy person (Angle and O'Donnell) or an ******* (Miller and Buck), and please both the establishment and Tea Party bases that will be able to campaign nationally.