* Republicans have controlled the executive branch for 8 years. The public tend to get antsy when one party is in power for too long in the executive (legislative is different, the GOP was in charge for 12 years and the dems were in charge for 40). Someone in the incumbent party winning after his party has had the presidency for 8 straight years is much harder. Al Gore had a harder time due to this fact, and he was running on the record of a competent, reasonably well liked president.
* Demographically the country is leaning democratic. Certain demographics tend to trend democratic (singles, non-religious, non-whites, young) and they are making up huge segments of the population. Virtually every segment that leans dem by huge margins like single women, youth, latinos, blacks, asians is becoming a bigger % of the electorate. In fact, single women (who vote dem by about 2-1 over the GOP) now make up 26% of the potential electorate. Youth now make up 1/4 and will be 1/3 by the next decade. At the same time demographics that lean GOP (financially secure, older, white, christian) are shrinking.
* Obama runs a much better ground game. he has more offices and more volunteers. Just like his massive win in the primary, he knows the rules of the game and where to put the pressure. He knows where to focus his energy and time, even if it isn't apparent on the surface. That is how he beat Clinton (who was considered a shoe-in).
* Democrats have been registering tons of voters. The democrats won massively in 2006, it was one of the handful of landslide elections in the US's history that reshaped politics. Since 2006 and that massive win (gaining 30 seats in the house and 6 in the senate is a rare occasion) over 2 million new democrats have been registered and 344k republicans have de-registered from that party. And that is just in the 28 states that keep track of voters by party. it is safe to say the same is happening in the other 22 states.
http://www.cbsnews.com/stories/2008/09/06/politics/main4422449.shtmlSome states (like my home state of Indiana) do not keep records by party. But in 2006 we had 3 house seats (out of 9 total) switch from R to D. In 2004 Kerry won about 950k votes. In the 2008 primary (where turnout is lower than the general) Obama & Clinton won 1.3 million votes combined. That means that between 2004 and 2008, an extra 350k people in Indiana were willing to vote for a democrat for president (and that 1.3 million fig is from the primary where turnout is lower). Some were limbaugh operatives, but its safe to say 250k+ new dems are registered in Indiana. So I would wager that since 2006 about 4-5 million voters have registered with the intent to vote dem in 2008.