The democrats aren't going to be nearly as enthusiastic about him as they were before when he had a more rock-star-like status. A lot of them feel he didn't deliver on a number of promises and many, especially the 20 something moderates, are rather unhappy with some of his more totalitarian moves. Those same policies will likely drive away a lot of the libertarians that would have otherwise considered him to avoid the republicans' more totalitarian social policies.
That said, the republicans have gotten far more extreme and alienated many of the economically conservative crowd as well as the people that are only moderately conservative on social issues. If the republicans go with Romney, they'll have trouble building excitement among the evangelicals and people that would take issue with some of his less right wing stances. Newt obviously brings a lot of baggage with him and isn't well liked among many republicans. Santorum would likely excite the most extremist of the right wingers and evangelicals, but fail to motivate the more mainstream republicans and cause most moderates to end up voting for Obama. While a lot of libertarians have sided with the republicans because of a dislike of Obama or because they were pleased to see Ron Paul and briefly Gary Johnson running on the Republican side, the extreme social stances are likely to drive away a good portion of them come election time.
It'll be an interesting race either way. I suspect Obama still has the upper hand though. Some of his worst policies simply aren't as well known as the various things that will drive voters away from the Republican candidates.
I had to cut a paragraph about my personal take on the candidates to fit within the maximum length, but to summarize it, I dislike all the candidates, Obama included. Santorum strikes me as the worst because of his extremely unethical positions and propositions that he tries to justify with religion.