3. Erie County, Penn. (pop. 269,728)
Outlook: Lean Biden

Philadelphia and Pittsburgh hog the spotlight, but Pennsylvania’s electoral ground zero might be its far northwest corner. The lakefront city of Erie is famous for assembling locomotives and packaging Smith’s meats. After decades of layoffs, it’s now home to a large population of refugees and the nation’s largest medical school by enrollment, Lake Erie College of Osteopathic Medicine.

Erie was also the site of a big Trump breakthrough in 2016: He carried the county by nearly two points, defying decades of Democratic dominance owing to a strong union heritage. Mr. Trump likely needs to win it again to keep Pennsylvania in his column. A February Mercyhurst University poll showed Mr. Biden leading Mr. Trump 48 percent to 44 percent in Erie County, and that was before Covid-19 made headlines.