In the runup to the election, a number of election analysts noted that, despite polls showing a likely Republican takeover of the Senate, Democrats could perhaps take hope in the possibility that the polls were wrong, systematically biased toward the GOP.
In The New Republic, for example, Sam Wang, co-founder of the Princeton Election Consortium, wrote in October that "although Republicans have the advantage in polls, Democrats' track record of outperforming polls works in the other direction. For the moment, there's a decent probability that polling nerds will be surprised on November 4." Worried Democrats could hold out hope that the polls were wrong. "When errors occur, the outcome tends to be more favorable to the Democrat," he wrote.
As it turns out, the polls were wrong. They dramatically favored Democrats.
As polling guru Nate Silver writes at FiveThirtyEight "the pre-election polling averages (not the FiveThirtyEight forecasts, which also account for other factors) in the 10 most competitive Senate races had a 6-percentage point Democratic bias as compared to the votes counted in each state so far." (This doesn't account for Alaska, which takes longer to report election returns.)
This shows the danger of putting one's faith in the hope that polling data is wrong, as many conservatives did in 2012 with Mitt Romney and as some liberals clearly did this year. Yes, of course, it could well be wrong. Polling isn't perfect, and it often misses important trends. But if it's possible that the polls are systematically wrong, then it's possible that the polls are systematically wrong in a way that doesn't favor the party you favor. And for Democrats, that seems to have been what happened last night.