From The Fix at the Washington Post: Americans think the Republicans will win control of the Senate. See also the New York Times’ Upshot, which references this paper by David Rothschild and Justin Wolfers. In some sense, by asking me who I think is going to win an election you’re looking at not just who I’m going to vote for but who I think my friends are going to vote for, from talking to them. For example, if hypothetically I’m part of one party’s base but I know a lot of swing voters, I might think of who my swing-voting friends say they’re going to vote for and say that that candidate will win.
Essentially you’re inviting me to construct an ad hoc estimator of how the election will turn out by observing my social network. My own voting behavior is a biased estimator of the final election results; explicitly inviting me to think about what will happen invites me to remove that bias.
God plays dice 
Very good article. I’m facing some of these issues as well..