There's been and will continue to be a lot of hair-tearing over what happened last night, but the simplest factor in the 2014 midterms is one that no mainstream commentator is going to address:
The Second-Term Midterm (Almost) Always Sucks for the Party that Holds the White House. Period.
The exception to this rule: 1998.
More below the fold.
Again, midterms almost always suck for the president's party. See also: 2006, 1986, 1974.
2006 was the midterm in George W. Bush's second term. Both Houses flipped to the Democrats, which shocked many commentators because while the House was expected to flip, the Senate wasn't. In retrospect, this can also be traced to Republican overreach, such as the attempts to privatize and thus destroy Social Security that took up so much of the GOP's time starting in December of 2004 and lasting until 2006.
1986 was the midterm in Ronald Reagan's second term. The Senate, which had gone Republican in 1980 (yes, all the crap the GOP spewed in 1994 about "decades of unbroken Democratic control of Congress" was just that: crap), flipped back to the Democrats, and the Democratic House padded its majority margin. (Iran-Contra had just hit. You remember that scandal? It's the one the mainstream media buried because it interferes with their depiction of Saint Senile as all-popular.)
The one exception in recent years was 1998. Know why that was?
That was because of Republican overreach (namely, constantly trying to impeach Bill Clinton). Newt Gingrich was saying that the GOP would get a dozen Senate seats and 60 House seats because of The Hunting of the President. Instead, the Senate was a wash and the GOP House margin dropped five seats.
This is why we hear people like Bill Kristol saying things like "now the Republicans have to show that they can govern". They're begging the new GOPers not to do something stupid, like shut down the government next month.
That is all.