With all due respect to Nate Silver and FiveThirtyEight, the midterm polls aren't very good (and he did miss the O'Donnell primary win). Not that there is no information to be gleaned from polls, but they weren't necessary to project most of the individual winners in 2004, 2006, or 2008. It was a somewhat different story in 2002 when the midterm polls suggested that Democrats were going to suffer serious losses and they did.
There are five hundred thirty races and generally the quality of the candidates determines the outcome.
A couple of caveats:
If person in the White House is strongly favored or disfavored, he/she is a factor in individual races.
Publicized and major instances of corruption or dirty-dealing scandals by either party is a drag on candidates of that party.
Right now, much to the frustration of the tea-baggers, Obama is not strongly disfavored. He won by too large of a majority and his performance to date hasn't been poor enough for him to sink that much that fast.
Neither party is currently suffering under a banner of corruption. (Mostly because coziness with Wall Street, drug and insurance companies or military contractors isn't perceived as scandalous by voters.) Scandals that break after September have to be really big to have a major impact.
So, it looks as if the individual candidates within the limitations of their electorates will drive the 2010 election results.
Agree with Nate Silver on the North Dakota Senate race. John Hoeven is a dream candidate -- no polls needed for this one. But is Blanche Lincoln really "dead?" Are incumbents really as challenged as the media keeps telling us they are?
How does Barbara Boxer compare with Carly Fiorina?
Set aside your bias and prejudices, and watch the first few minutes of the Boxer-Fiorina debate with the sound OFF.
What did you see? Did you see a winner and a loser?