The least few days I have read about 20 diaries with one of two theories being floated: 1. The Democrats lost because they betrayed liberals. 2. The Democrats lost because left-wing bloggers attacked Obama. Here is the problem with these explanations: there is no data to support either one.
Was turnout low among young people? In 2006 it was 12% of the electorate (2006 was a good year for Democrats). This year it was 11%, not really statistically different. Turnout was way down over 2008 (it was 18%) but turnout in that group always goes down in off-year elections. Moreover, turnout in that group was large enough to yield large gains in 2006.
By the way: in 2004 youth turnout was 17%. I don't remember anyone talking about the decline in youth vote here in 2006.
The African American vote was 10% in 2006, and it was 10% in 2010. It was 11% in 2004. I wonder why it went down from 2008?
Moreover, despite either the "betrayal of Democrats by Obama" or the "betrayal of left-wing bloggers" meme Democratic Percentages among liberals and Democrats WENT UP over 2008. In this election, Democrats carried 90% of liberals, and 92% of Democrats (versus 89 in both in 2008). Moreover, the percentage of liberals in the electorate was the same in 2006 as in 2010 and not materially different in 2008.
Democrats also did better among liberals than in 2004 (85%) and in 2006. In fact, the 90% of liberals that the Democrats received among liberals was the highest they have received since at least 2000!!
In short, I think both sides of this argument aren't really looking at the data. The idea liberals didn't vote isn't supported by the data, nor is the premise that they didn't support Democrats. This election was about independents and in my view the economy (since 48% wanted to repeal HCR, and 47% didn't)