I put a lot of stock in this poll, it's got a tremendous track record. That said; I have no explanation for a series of polls, including this one, that show this race tightening. What has Burke done that would see this close? What has Walker done to jeopardize a previous 7 point lead? Nothing and nothing, in my opinion.
Burke's campaign has been completely incompetent and she has spent little thus far, which makes this poll, well, terrifying to me. And the Journal Sentinel is likely right; this is going to lead to a lot of outside money pouring into this race, on both sides.