Fivethirtyeight.com's performances

[pbrower2a]

The 2008 election was never a stable situation. Those who followed the polling trends saw  states like Michigan and North Carolina make wild swings during the summer. By September (immediately after the Republican National Convention) the Presidential nominees were about even in popular votes even though Obama had a huge structural advantage -- a bunch of states, nearly enough to win with a bunch of states in easy reach for Obama.

Poll results were unstable because situations were changing. Places that seemed to get close got the attention of McCain (not too bad) and Palin (dreadful). States that the GOP thought vulnerable (like Michigan, Wisconsin, New Hampshire, and Iowa) suddenly went out of reach. So McCain/Palin went after the states that were close but had to be won -- like Colorado, Virginia, Ohio, and Florida. Palin got overexposed in Colorado and Virginia, and that likely ensured an Obama victory.   

In the end, one thing that was certain but whose timing wasn't -- the economic meltdown -- created an electoral disaster. Most people thought New Mexico and Nevada would be close, and they weren't.