Actually, Wisconsin is actually very blue, very red and very purple. The city of Milwaukee and the People's Republic of Madison have been and always will be blue. Western & Northwoods Wisconsin is bluish-purple (I blame the proximity to Minnesota DFLers). The suburban/exurban counties surrounding Milwaukee are extremely red. and central and northeastern Wisconsin are moderately red.
Yes, Wisconsin has not been won by a GOP presidential candidate since 1984, but that is largely due to rampant voter fraud in Madison & Milwaukee. In lower turnout non-presidential years, the state as a whole is purple with a slight reddish tinge.
With regard to this specific poll, the poll oversampled Democrats +5% and conducted the poll over a weekend which is viewed by polling experts as problematic. Weekend sampling tends to produce more tepid GOP responders.
I'm not worried. No matter what polls may say, Walker will win 53-47.
Good information, thanks!