The pollsters at Gallup include this analysis:
It's likely that the 2008 election will be fought in the battleground states, just as in prior elections. Gallup's election polling to date suggests that the presidential election could be very close, because neither McCain nor his Democratic rivals have maintained much of a lead in recent weeks.
The analysis of competitive states adds insight into how the candidates are doing beyond the overall vote figures. A candidate must do reasonably well in those states to have a chance at winning. For example, McCain could open up a significant lead in the national vote based on a very strong performance in the red states, but that would not mean he was better positioned to win than if he were not doing as well in the red states but doing better in the more competitive states.
As of now, Obama and Clinton have an advantage over McCain among voters in the competitive states. Given that more states fall into the Republican column than into the Democratic column, the Democratic nominee probably needs to maintain that advantage in order to prevail in enough purple states to gain the electoral vote advantage in November.