According to FiveThirtyEight:
“If the polls are right, Republicans will take control of the United States Senate when it reconvenes next year. They’ll retain the majority of the nation’s governorships, although perhaps with a net loss of one or two seats. FiveThirtyEight hasn’t issued a House projection this year, so here goes nothing: Republicans will keep it. (Want more detail? The Cook Political Report thinks Republicans will gain a few seats, probably a net of 6 to 12, from Democrats.)
“Sounds like a pretty good night for Republicans? It would be. But there are a few apparent complications.”
Complication No. 1.
Some prominent Republican incumbents are likely to lose. Senator Pat Roberts of Kansas is no better than even money to keep his seat against independent Greg Orman. Incumbent Republican governors are underdogs — some by slim margins — in Alaska, Florida, Kansas, Maine and Pennsylvania. Rick Snyder of Michigan and Scott Walker of Wisconsin are likely but not certain to survive.
Complication No. 2.
Republicans are largely playing on “home turf.” The average Senate race this year is being held in a state where Barack Obama won just 46 percent of the vote in 2012. In the House, meanwhile, the median Congressional district is Republican-leaning. (Democrats tend to be packed into geographically compact, urban areas; this tendency is sometimes enhanced by gerrymandering.) A method of assessing the score probably needs to account for this.
Complication No. 3.
The House, Senate and gubernatorial results seem to tell different stories. Polls project major Republican gains in the Senate but modest ones in the House and perhaps a net loss of Republican governorships. How to reconcile this evidence?