Considering there's no trend line indicating any downward movement and this is quite a sharp shift, my gut says outlier. The poll being summer polling strengthens that thought.
However, we'll really know (for sure) in the next couple of months.
He has actually been going downward 53-51-48-43. This month did see the sharpest drop, but he has been headed down. Granted the internals tend to have higher margins of error, but the Senate campaigns getting under way could have hurt him, as he saw a rather sharp drop among Democrats
The last Rasmussen poll had him ahead of the Franken and Ciresi guys by 10%. This was a few months ago. With his approvals down and the good fundraising by Franken, they might be about tied now ...
