I agree with Sean Trende that, even though the polls in most swing Senate races are very close, the Republican Senate candidates should, absent freak occurrences, win most of the competitive races.
Another way to look at it is in terms of presidential job approval. At this point in 2010 (a very good year for the GOP), Obama’s Real Clear Politics average job approval rating was 46 percent and his disapproval was 48.1 percent. Obama’s current job approval is 41.8 percent and his disapproval is 52.4 percent. At this point in 2010, the median respondent was ambivalent (neither approving nor disapproving) of Obama while today a clear majority disapproves.
And As Trende pointed out, most of the competitive states have Obama approval ratings that are comparable to, or lower than his national approval rating. That means that the voters who will decide the election will overwhelmingly ones who think that Obama is doing a bad job and it is doubtful that the median Alaska, Arkansas, Iowa, Kentucky, and Colorado Obama disapprover is a pro–tax increase abortion enthusiast who is frustrated that Obama didn’t ban all guns and establish single-payer health care by executive order.
It is possible for a Democrat to win in that kind of environment primarily by distancing themselves from Obama. Joe Manchin did it in 2010, but I don’t think that many of the Democrats stuck in very close races will pull of the same trick. They don’t have the same strength of standing as moderates (especially incumbents who voted for Obamacare, their opponents are generally better fits for the state than Manchin’s challenger and those states have a more recent history of sending Republicans to the Senate.
All bets are off if Democrats can produce an entirely different electorate than that of 2010 through their voter turnout operation or if Obama can lift his job approval ratings but, as things stand, The Republicans should be better off than you would think from looking at the head-to-head polls.