This graph shows the electoral college score over time counting only those states where one candidate has a lead of at least 5%. The states where it is very close don't count here. With one exceptional period, it is possible to draw a pretty clear straight line through the middle of the blue line from mid-April until now. The exceptional period occurred in the last two weeks of September, when McCain got a small bounce from Palin's incendiary convention speech, which really fired up the base. However, when the bounce faded, as all convention bounces do, Obama continued on his upwards trajectory. If you compare 2008 to 2004 (on the Electoral coll. page) you see that this year looks completely different from that one. Other than a two-day blip on June 24-25, 2004, when Kerry briefly touched 300 EVs, neither candidate ever was even close to 270 EVs in the solid states although Kerry was above 270 counting the "barely states" until his Swift Boat was torpedoed.
So what's with the nearly straight line upwards for Obama? Charlie Cook and many other nonpartisan experts have said that with 80% of the population saying the country is on the wrong track, this was going to be an election about Barack Obama. People clearly want change, but they were not yet sure if Obama was up to the job. After 18 months of campaigning and countless primary and general election debates, people are getting to know Obama and are beginning to feel he is a good man in a storm. His calm behavior during the bailout bill period--and McCain's erratic behavior--helped Obama a lot (see graph). Slowly but surely people are coming to believe he can do the job, and that's all he needed.