There are unquestionably many factors behind this result. But I want to focus on the biggest one that was completely under Democrats' control. It is the same thing that killed the Republicans of Hoover's generation: gross mishandling of an economic crisis. Democrats had the full run of the federal government from 2009-10, during the worst economic disaster in 80 years, and they did not fully fix mass unemployment, nor the associated foreclosure crisis. That is just about the most guaranteed route to electoral death there is.From the beginning, the Obama has been more concerned with coddling Wall Street, as opposed to dealing with problems that the banksters caused for Main Street.
In terms of the foreclosure crisis, they actually aided and abetted the criminals. (See Geithner, Timothy, "Foaming the runway.")
Much like Hoover, they continued to insist that the economic orthodoxy that caused the economic meltdown was the solution to the problems.
Obama actually went softer on corporate criminals than George W. Bush.
Is it any wonder that the Democratic Party is a political Super Fund site right now?