What if the polls prove to have no bias? Our model shows Republicans as about 75 percent likely to win a Senate majority. This may seem confusing: Doesn’t the official version of FiveThirtyEight’s model have Republicans as about 60 percent favorites instead? Yes, but some of the 40 percent chance it gives Democrats reflects the possibility that the polls will have a Republican bias. If the polls were guaranteed to be unbiased, that would would make Republicans more certain of winning.

Nate Silver