For much of this election cycle, Democrats complained the polls were biased against them. They said the polls were failing to represent enough minority voters and applying overly restrictive likely-voter screens. They claimed early-voting data was proving the polls wrong. They cited the fact that polls were biased against Democrats in 2012.
The Democrats' complaints may have been more sophisticated-seeming than the "skewed polls" arguments made by Republicans in 2012. But in the end, they were just as wrong. The polls did have a strong bias this year — but it was toward Democrats and not against them.
Based on results as reported through early Wednesday morning — I'll detail our method for calculating this in a moment — the average Senate poll conducted in the final three weeks of this year's campaign overestimated the Democrat's performance by 4 percentage points.