"This is a big 'skew,' and it comes on the heels of what had been a fairly substantial bias in the opposite direction in 2012," Mr. Silver wrote. "The polls ... are having some problems."
"Probably a lot of incumbents from both parties lose or come close to losing. So how do you square this kind of moderately pro-Republican mood with the very anti-incumbent mood? Could you have a Republican like Pat Roberts in Kansas lose, even though Republicans have a good night overall? So it's a multidimensional sort of election. Makes it a bit more complicated," he said.