Silver compares 2014 to past elections:
The data point for 2014 reflects this year’s generic congressional ballot, a poll-based projection of the national House vote. There’s been huge variation from survey to survey on the generic ballot, but the average currently favors Republicans by about 2 percentage points. If the average is about right, it would tell us the same thing we inferred a few paragraphs ago: 2014 is a better year for Republicans than 2012 and a much better year than 2008 but not as good as 2010.
So maybe this isn’t so complicated, after all. The polling data we have tells a pretty consistent story. The challenge is in the different interpretations it will enable, all of which will feature prominently in the post-election spin.
Sam Wang largely blames the gains Republicans are likely to make in the House on gerrymandering:
The House is, in many ways, a predictable game: a modest gain in the popular vote will very likely lead to even further seat gains. Roughly speaking, gaining one percentage point in a popular-vote victory should translate to approximately three more seats. A popular-vote tie would lead to a gain of four seats, and a two-percentage-point win would lead to a gain of ten seats. If the G.O.P. gains eight seats—well within the realm of possibility—they will make up all their losses of the 2012 election. This would put the House back to where it was after the election of 2010, a so-called wave year, when voter opinion swung strongly to the right. Using the tool of redistricting, they have successfully tilted the political playing field to secure a large majority for at least the next two years without the same popular appeal.
The governor’s races are a different story. Wang checks in on them:
At this point, races fall into the following categories:
Incumbents headed for probable defeat (>3 percentage point margin): Brownback (R-KS), Corbett (R-PA).
Incumbents under threat (<3 percentage points): Parnell (R-AK), Deal (R-GA), Snyder (R-MI), Walker (R-WI), LePage (R-ME), Scott (R-FL), Quinn (D-IL), Malloy (D-CT), Hickenlooper (D-CO).
Open governorships, clear lead (>3 percentage points): Raimondo (D-RI), Baker (R-MA), Hutchinson (R-AR).
The expected net range of outcomes (1 sigma, about 68% of possibilities) is D+0.4 ± 1.3 governorships, which translates to between 1 net gain by Republicans to 2 net gains by Democrats.