It's only the beginning of an explanation. It's self-evident that incumbents are punished if things don't go well. The issue here is whether the Democrats as the governing party could have taken steps to make things better - or to at least go less badly. That's the issue which liberals and those to their left have been addressing. Their argument is that the Obama administration could have confronted the Republicans and pursued policies which would have sustained its popularity. Their model is the Roosevelt administration, which immediately and effectively moved to restructure the banking industry, create jobs, and prevent foreclosures in open defiance of the "monied interests" and "malefactors of great wealth" on the right who were frothing about its "socialist" policies. The Obama administration balked at going down this road and working class standards continued to deteriorate, so naturally as the incumbent party presiding over a continuing slide, it stood to be punished by the voters, including many of those who supported it two years earlier. No surprise there.
^^^^^^^^ CB: The situation today is not the same as it was in 1933. The Depression unemployment rate was higher than it was/is in the current recession. Then the Depression had been going on since 1929, longer than today's recession "had been going on". There was no unemployment insurance, welfare, Social Security, Medicaid, Medicare to cushion the impact. The significance of this is that people were more desperate economic straits in 1933 than today, so there was a larger popular basis for desperate measures then than there is today. Roosevelt was a member of the bourgeois and white, and therefore didn't face a crazed racist onslaught in response to his measures - a fact that you continue to fail to include in your analysis thereby weakening it considerably. These facts significantly weakened Obama's ability to forcefully lead the Democrats to do as you say. It made the Blue Dog Democrats into stiff opponents of the most progressive social democratic steps. (In fact, even FDR compromised with the Southern Jim Crow Democratic Senators in shaping Social Security so that it didn't go to many job types held by Black people.) Then there was a growing movement to organize industrial unions; now labor is in decline. The comparison to 1933 doesn't wash.
^^^^^^^ "it stood to be punished by the voters, including many of those who supported it two years earlier. No surprise there. "
^^^^^^^ CB: It was largely so-called Independents who switched on the Dems. "Independents" are types susceptible to Tea Party rhetoric not social democratic rhetoric. You have not demonstrated that those who voted Dem last time and Rep this time did not do so because they bought the Tea Party line, in other words for the opposite reasons that you give and that they did not want your larger social democratic program. Given the Reaganite/libertarian/anti-social spending ideology that is rife in the US population today, it is very likely that the Independents shifted because they didn't like the social democratic aspects of "stimulus" and healthcare reform.