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.
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