The determinant of all this is the economy, as Brendan Nyhan has argued all year. This post summarizes hie perspective well:
I'm bracing for an avalanche of nonsense tomorrow night about why Barack Obama is responsible for the expected Republican landslide. Here's a guide to what you should expect.
It's long been obvious that Obama's political standing would decline as a result of the poor economy and the passage of time. Similarly, substantial Democratic losses in the House were always likely given the large number of seats the party had to defend in a midterm election in which it controls the presidency. The continued weakness of the economy subsequently appears to have enhanced the Republican advantage, helping to produce tomorrow's pro-GOP wave.
Instead of focusing on these structural factors, journalists and other political figures have constructed a staggering number of ad hoc claims about messaging, tactics, etc. to "explain" what has happened to Obama and the Democrats:
-Obama's message is not populist, thematic, simple, and/or comprehensive enough;Etc., etc.
-Obama failed to "connect" with voters (in part because he often uses a Teleprompter);
-Obama has an "empathy deficit";
He goes on to list many more examples of what we can we can expect to hear.
What matters going forward will be, as always, what happens to economic growth over the next two years.