Barack Obama won the national popular vote by about 7 points in 2008. Republicans look to have won the national House vote by about 7 point this year. One big question about that 14 point shift- how much of it was Democrats going to the Republicans and how much of it was Democrats staying home?
The answer is about half and half. The national exit poll showed those who voted Tuesday split their votes for President evenly in 2008, 45% for Barack Obama and 45% for John McCain. If you take the breakdown of how they voted and weight it to the actual 53-46 win Obama had in 2008 you get Republicans up by just a single point in the House vote, 49-48. So about 8 of the points of movement in 2010 can be attributed to voters changing sides (13% of Obama voters chose Republicans while only 7% of McCain voters chose Democrats) and the other six can be attributed to the Democratic turnout dropoff.
Those numbers have to be at least a little reassuring for Democrats. Most of those drop off voters are probably going to be back in 2012, especially after a couple of years watching how the Republicans in the House handle themselves. The fact that Democrats would be running even nationally at this very low point for their party if their folks turned out bodes well for the next election if there's even a small amount of warming up to the President between now and then.