So why were the polls so incredibly wrong?: Huge bias in favor of the Democrats

What was supposed to be an easy Democratic victory in Maryland ended up being a Republican landslide win by nine percentage points.  In Illinois the Democrat was supposed to win, but the Republican won easily.  In Kansas what were supposed to be a couple close races turned into Republican comfortable victories for the Senate and governorship.  In Virginia, a race that no one thought would be remotely close turned into a late night nail biter and still isn't quite over.  Democrat Alison Lundergan Grimes in Kentucky was supposed to be fairly close to Mitch McConnell, but it turned into a 15 percentage point tidal wave against her.

Larry Sabato:

"I want an investigation of the polls in Virginia," University of Virginia political scientist Larry Sabato told Fox News. "They were completely wrong, just as they were in Georgia. They were also way off in Illinois. And I could go on and on.

 From Nate Silver:

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. The average gubernatorial poll was nearly as bad, overestimating the Democrat’s performance by 3.4 points. . . .  

Interestingly, this year’s polls were not especially inaccurate. Between gubernatorial and Senate races, the average poll missed the final result by an average of about 5 percentage points — well in line with the recent average. The problem is that almost all of the misses were in the same direction. . . .

A possible explanation is provided by Politico:

Republicans nearly swept the 10 Senate races rated as toss-ups by the Cook Political Report on Election Day, winning seven of the 10 outright – they lost New Hampshire, Alaska remains undecided and Louisiana is going to a runoff. 

To be sure, Republicans were favored to win back the Senate on Tuesday. They led in six of the 10 contested races – and hold the advantage in a seventh, Louisiana, in the runoff next month.

But it wasn’t that Republicans won so many of the most competitive races — it was how much they won by. . . . 

Republicans have long claimed that public polls, usually conducted by randomly dialing phone numbers rather than only contacting voters with a history of turning out in midterm elections, include too many people who won’t ultimately cast a ballot – a group that tends to lean Democratic. 

Those public surveys, they say, also weight, or peg, their demographic data to known Census parameters, ignoring historical trends of the midterm electorate – which is usually older and more white. 

“I think the media polls were dramatically off because too many media pollsters use Census weights,” said Republican consultant Brad Todd, whose firm, OnMessage Inc., conducts polls and creates TV ads for GOP candidates. “In a midterm electorate, using the Census as a reference point would have the same value as using a grocery list as a reference point.” . . . .

Nate Silver gave Republicans just a 15.6% probability of getting 54 seats in the Senate.