The President is selected by the number of electoral votes they receive. The campaigns will focus on getting electoral votes. 270 are needed to win.
Currently state by state polling shows, Obama has 226 where he is expected to win and McCain 189. 123 are up for grabs in 11 states. McCain will have to pick up 81 and Obama 44 to win. Both campaigns will focus time and effort in the states they feel are most likely to bring them the electoral votes to win.
Polls which indicate a national average are interesting but it's the state by state polls which indicate which candidate is leading.
Here's one such poll, the CNN electoral vote polling. Other's may differ but from what I have seen not by much.
CNN's Electoral Map: Who's ahead - CNN.com