A Surging Virus. Angry Packers Fans. Can Trump Hold On to Wisconsin?

In Democratic strongholds like Milwaukee and Madison, the state’s two biggest municipalities, the margins actually improved slightly for Democrats between the 2012 and 2016 presidential elections. Dane County, which includes Madison, is the state’s fastest-growing county, and is probably the No. 1 area where Democrats will be looking to run up the score.

And with Wisconsin’s heavily white population growing older, thanks to the aging of the baby boom generation, Mr. Trump’s underperformance among voters 65 and older in polls this year could help Mr. Biden across the state.

Green Bay, a heavily blue-collar city in the northeastern part of the state, has been in the news recently as the coronavirus has surged. The Packers, the town’s beloved N.F.L. team, announced this week that for the time being no fans would be allowed to attend games at Lambeau Field. Data released by the N.F.L. Players Association at the end of last month showed that Green Bay had a higher rate of infection than any other team’s market.

For Mr. Trump, all of this puts an unwelcome focus on the coronavirus in the most politically volatile region of the state.

“Green Bay, Appleton and other cities in that region have moved pretty noticeably in a Democratic direction since 2010,” Dr. Franklin said. “The surrounding counties in the region, though, have stayed very Republican, and to the north and west of Green Bay have become even more Republican than before. So the result is, the whole region is still pretty competitive.”

Brown County, which includes Green Bay, broke for Mr. Trump by nine points in 2016, but with Mr. Biden turning back Mr. Trump’s advantage among suburbanites and building his support among the urban Democratic base, it could be up for grabs this year.

Over all, Wisconsin’s population is almost evenly split between Democrats and Republicans, but there’s still a slight Democratic tilt to the electorate, especially when voter engagement is high, since Democrats tend to do better among demographics with lower turnout.