Presidential elections tend to be a referendum of the incumbent and the job s/he may have done. By that metric, President Obama should be in trouble given the poor state of the economy.

That said, the main way people  vote against an incumbent is to vote for the candidate of the other major party.  But for people to vote for the challenger, they need believe that there are at least some issues the challenger will be able to handle better than the incumbent. People will reject the incumbent, but only if they think that could lead to some improvements. It appears Mitt Romney is currently failing to cross even this rather law bar. From Pew:

The one issue Romney is seen as better than Obama on is the federal deficit, beyond that Obama outperforms him on everything else. Obama beats Romney on the economy issue of creating jobs and the social issues like abortion. This is probably why the poll found Obama is leading the general election match-up with 50 percent, compared to Romney who is at only 43 percent.