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Here's the data set in the article illustrating the fact that who's ahead depends a lot on whether you're looking at partisan polls from each party of non-partisan polls. As Cohn describes it, "The possibility that partisan polls could skew the averages is perhaps best illustrated in Arkansas, where partisan polls are breaking along predictably partisan lines."
But wait a second, let's look more closely. The Democratic polls and the non-partisan polls are nearly identical. And actually, you only get that number by including two polls from Rasmussen. They may be a bit less problematic since Scott Rasmussen left the company. But by any reasonable measure they count as a partisan pollster. If you remove Rasmussen from the non-partisan set the non-partisan number balloons to Pryor +8.
So, without putting too find a point on it, the Dem-leaning 'partisan' polls seem right in line with the non-partisan polls. They actually seem to trail them in showing the extent of Pryor's lead. It's really Dem-leaning polls and non-partisan polls on one side and Republican polls on the other.
Not every race is going to be like this certainly. And just eye-balling it, it's a little hard for me believe he's quite as far ahead as NBC/Marist seems to show. But still. This data set doesn't really capture what it purports to show and I think the pattern - that this is more an issue with Republican pollsters than Democratic ones is very real.