LOUDOUN COUNTY, Va.—The beer was flowing, the handmade potato chips were self-serve, and hope was in the air. Early in the night, the Loudoun County Democrats who gathered at the Döner Bistro in Leesburg were cautiously—anxiously—optimistic: Sure, it had been a rough year. A global pandemic, regular protests at the local school- board meetings, and the contentious governor’s race, rife with misinformation, had pitted neighbor against neighbor. But these volunteers had done the work. They were confident that Democrats could pull off the first victory of the midterm cycle and set the stage for next year’s elections. “The commonwealth moves forward, not backwards,” Lissa Savaglio, the president of the group, said from a stage at the front of the restaurant. “We’re not interested in repeating history.”

The voters of Virginia had other ideas, handing the Republican private-equity executive Glenn Youngkin the governorship one year after helping deliver the presidency to Joe Biden. Despite Savaglio’s hopes, the GOP...........