Sean Trende, the senior elections analyst for Real Clear Politics, has been looking at the issue of screening for likely voters and has come to the same conclusion I did when looking at polls in Washington state: some pollsters are likely including too many Democrats in their weighted samples.
Trende argues that despite all the evidence of a big Republican advantage in turnout, in developing their likely voter model, many pollsters are seeing an electorate that looks like the past two elections, and he cites the U.S. Senate race in Washington to make his point:
But it doesn't appear that the screens being used by pollsters this year are creating an electorate that resembles 2004 at all. Instead, the partisan environment these pollsters' screens are producing is somewhere between 2006 and 2008 — the type of electorate you'd expect to see in a good Democratic year.
A quick examination of the crosstabs indicates that these pollsters are almost all seeing the same basic environment in race after race: Republicans and Democrats are drawing near unanimous support from their partisans, while Democrats are drawing support from 35-40 percent of Independents.
In other words, these pollsters don't disagree on how Republicans, Democrats and Independents are going to cast their votes. Instead, they disagree on how many Republicans, Democrats and Independents are going to cast their vote.
Two recent polls in Washington State provide a perfect example. Consider the following table:
As you can see, the two pollsters agree nearly 100 percent on how Republicans, Democrats, and Independents are going to vote in this race. This means that the difference between CNN/Time's finding that Murray is ahead 8 points and above 50 percent, and PPP's finding that Murray is ahead 2 points and below 50 percent, is almost entirely attributable to differing numbers of Republicans, Democrats, and Independents making it through the respective pollsters' screens.
I have seen enough now to be confident that Elway, CNN, and even SurveyUSA, are oversampling Democrats, and that the recent Moore, PPP and Marist polls are showing an accurate picture of the race between Rossi and Murray. Both candidates are just below 50 percent and the race is within the margin or error. This race is a coin flip with one week to go.