Many in Wisconsin are shocked that Tom Barrett somehow went from being up by a point to being down by six points in a span of only two weeks. Have voters suddenly taken a sharp disliking to Tom Barrett? No.
The poll numbers have changed because they significantly changed the sample of who they polled. In this poll there are more conservatives and fewer moderates and liberals:
Poll Two Weeks Ago: Conservatives 43% Moderates: 32% Liberals: 22%
According to Marquette's numbers, conservatives favor Walker 72% to 21%, moderates favor Barrett 50% to 43% and liberals favor Barrett 83% to 15%.
Not suprisingly, when you poll more conservatives and fewer moderates and liberals, you're going to have a very different result.
As discussed in previous posts, if we average the last three exit polls in Wisconsin (2006, 2008 and 2010), we find that the actual breakdown of the Wisconsin electorate is 22.7% liberal, 46.7% moderate, and 31% conservative.
In addition, as John Nichols is pointing out, the pro-Walker Milwaukee suburbs are "shockingly" over-sampled in this poll compared to the one they did two weeks ago.
Bottomline: Don't get caught up in the polling hype. This race is a dead heat and is going to come down to which side can get people turned-out to vote.