In the latest issue of Political Science Quarterly, Gary C. Jacobson analyzes what happened in '08--finding Democrats' party-identification advantage had much to do with it, and gives a big picture on 2010, warning that conditions might not be so favorable to Democrats next time around for some macro reasons.
As far as 2008 goes, Jacobson finds several factors played in Democrats' ability to pick up 21 House seats, 8 Senate seats, and the White House:
1. George W. Bush's massive unpopularity and the economy: as the Iraq "surge" began to succeed, the economy worsened, and Bush's chances for regaining some public standing were canceled out. Coincidentally, the messaging frame Obama sought to create--based on jobs and the economy--became more prevalent than John McCain's frame of national security. Notably, Bush's approval among Republicans dropped from 81 percent to 61 percent, and among moderate 31 percent to 20 percent, and GOP support eroded as, in Jabocson's words, "Republicans had stuck with the President when Iraq was the dominant issue, but the economic meltdown evidently proved to be the last straw for more than a few of them."
2. Party ID: party loyalty was consistent with its levels in the previous two presidential elections (90.5 percent in 2008, 91 percent in 2004, 90 percent in 2000), but Democrats held a 55 to 41 percent advantage in party ID in 2008. Despite the post partisan and/or bipartisan images both Obama and McCain sought to portray, party ID and party allegiances played a big role in the 2008 election