But there is one poll where Rasmussen has consistently polled all adults rather than defaulting to its likely voter model, on the issue of party affiliation. And that shows Rasmussen's clear bias towards the Republicans. Nate Silver explains:
The argument goes like this: those people who vote most reliably in midterm elections tend to be older, whiter, and to have higher social status -- which are also characteristics of voters that generally lean toward the Republican candidate. When coupled with what also appears to be a Republican enthusiasm advantage this cycle, it is quite reasonable to believe that a poll of likely voters (like Rasmussen's) should show more favorable results for the Republicans than one of registered voters or adults (like most others).
This argument is completely true, insofar as it goes. But it is not sufficient to explain the bulk of the Rasmussen house effect, particularly given that Rasmussen uses a "fairly loose screening process" to select likely voters.
In fact, this is quite readily apparent. Although Rasmussen rarely reveals results for its entire adult sample, rather than that of likely voters, there is one notable exception: its monthly tracking of partisan identification, for which it publishes its results among all adults. Since Labor Day, Rasmussen polls have shown Democrats with a 3.7-point identification advantage among all adults, on average. This is the smallest margin for the Democrats among any of 16 pollsters who have published results on this question, who instead show a Democratic advantage ranging from 5.2 to 13.0 points, with an average of 9.6.
To be clear, the partisan identification advantage among registered or likely voters is much smaller. A 3- or 4- point gap would be quite normal there. When making an apples-to-apples comparison to other polls of all adults, however, it is something of an outlier and would reflect a house effect of about 6 points when measuring the net difference between Democratic and Republican preferences.
Meanwhile, an increasing number of pollsters have begun to publish results among likely voters in their take on the Congressional generic ballot. Six pollsters apart from Rasmussen, in fact (these are GWU, Bloomberg, NPR, Democray Corps, OnMessage and McLaughlin) have done so since December. They show the Republicans leading the generic ballot by an average of 2.8 points among likely voters, on average (if explicitly partisan-affiliated polls are included, the margin is similar at R +3.3). This is a potentially excellent result for them -- one which might imply a massive, 50+ seat swing in the House, but is less than the 9-point advantage that Rasmussen now shows, and has shown consistently throughout this period.
Note that the house effect here, again, is about 6 points (the difference between the R+9 that Rasmussen shows and the R+3 that the other likely voter polls do). This is of the same magnitude of the 6-point house effect that was introduced in their construction of the all-adult sample, as described above. In other words, Rasmussen does not appear to be applying an especially stringent likely voter model. Instead, the house effect is endemic to their overall sample construction and is "passed through" to their likely voter sample.
In other words, Rasmussen is stone cold busted. Just about any Rasmussen poll has to be taken with a six point or more bias towards the Republican position. The numbers just don't lie, they strongly, strongly favor the Republicans in everything they do.
There's a reason why Republicans are constantly quoting Rasmussen polls as proof the Democrats are doomed. Nate's got them dead to rights, and the numbers prove it.
As far as I'm concerned, Rasmussen is a GOP polling outfit.
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