I want to point out that it's not 'Wisconsin' that is a Republican stronghold, it's literally just an college-town other-town divide and Wisconsin is gerrymandered to shit for it's elections, getting worse every year. This was mostly caused by the TPP in the 90's. Prior to the 90's Wisconsin was a Progressive-Democratic stronghold, but when the TPP destroyed the industrial rural labor unions, funding for local democratic party officials dried up while simultaneously the Supreme Court decision that allowed for unregulated super-pac money flowed from the Koch Brothers, turning most of those people, who would have traditionally been Progressives (not socialists, definite distinction) into Libertarians, which is the still the most popular strand of Republican thought in Wisconsin. Ron Johnson is a creature from that mileau, though he plays off of the Trump and traditional Republican bases well, and as such is deeply tied into the voting areas around Western and Northern Wisconsin. Mandela Barnes, on the other hand, is a creature of the democratic political machines still that exist by the skin of their teeth in most college and hospital towns in Wisconsin, playing better in Eastern Wisconsin and towns bordering Minnesota.

It's honestly a toss up. I think Johnson's going to squeak by with it as Barnes isn't enough of an outsider to actually stir up any real enthusiasm, but Johnson really put his foot in his mouth by saying he was going to 'put seniors back to work', so it's going to be a bunch of money wasted, regardless of the outcome.