Democrats began the search last summer, fanning out through the northern swing states of Wisconsin, Michigan and Pennsylvania to find the nurses, electricians, farmers and floor installers who could best testify to the danger of Donald Trump returning to power. The previously unreported project by the outside group American Bridge 21st Century developed 732 leads, conducted 472 interviews and then filmed about 50 voters who will anchor a $140 million ad campaign starting this spring, aimed at reminding women and working-class voters why they voted against Trump in 2020, according to the group’s leaders. “The hardest part about using the...