Hardly shocking. The GOP primary campaign has turned very negative while Baldwin is running upbeat ads trying to define herself. Of course she's not portarying herrself as the radical Madison liberal she is, which is also helping her cause. The eventual nominee will have time to move beyond the primary, but it appears they are dinging each other.
I'm not predicting this, but it will be interesting to see if Jeff Fitzgerald has closed the gap, assumping there's another poll before the election. Also, if it is the negative GOP race that's helping Baldwin, I'm not sure why Hovde does the best against her in this poll. He's getting attacked as much as anybody.