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
3. Turnout in key states: voter turnout rose nationally by 3.4 percent (it had risen 1.6 percent between 2000 and 2004), but it rose more in some key states for Obama, as Obama's mobilization efforts kicked in: 8 percent in North Carolina, 7.1 percent in Virginia, and 4.6 percent in Indiana.
4. GOP retirements and Democratic recruiting: after slipping into the minority in 2006, some GOP lawmakers retired, making it difficult for the party to defend their seats. At the same time, no Democratic senators and only six representatives retired (three to run for Senate), and Democrats recruited candidates who could win.
5. Money: the same electoral conditions that made '08 favorable to Democrats in general--most notably, Bush's unpopularity, led to successful Democratic fundraising efforts, and the party was able to outspend the GOP
As for 2010, Jacobson says things might not be so easy for Democrats the next time around. Jacobson notes that a quarter of Democratic House seats are in districts where Bush won more than 53 percent of the vote in 2004; picking up these conservative districts will not only have the effect of moderating the Democratic caucus--it will make those seats tougher to defend.
Similarly, Democrats won't benefit from an unpopular GOP president, and they now own the economy--if it's bad in 2010, it will be bad for them. Jacobson writes:
Democrats have had the wind at their backs in two successive elections, but now that their Party bears full responsibility for the government's performance under the most difficult circumstances faced by any incoming president and Congress since the 1930s, they cannot expect political conditions to favor them a third time running; the contrary is much more likely...
The set of seats up in 2010 should help Democrats retain their Senate majority, because only 2 of the 16 seats Democrats will defend are in states won by John McCain, while Republicans must defend 19 seats, 6 in states that went to Obama. On the House side, however, Democrats will have to defend seats in 63 Republican-leaning districts (Table 6), including 16 that had given Bush more than 60 percent of the vote in 2004. The Republicans' structural advantage in House elections--even greater now than it was back in 1994--provides the foundation for a major Republican comeback should the Democratic regime falter.