It was four months and eight days until the 2018 midterm elections, and Anna Eskamani had not spent a penny to promote her political campaign online. Ms. Eskamani, 27, is running to represent Florida’s 47th District in the State Legislature. Like many state-level candidates, she writes her own fund-raising emails and manages her own social media accounts. And with her busy schedule on the campaign trail, advertising online is an afterthought. “My budget is zero for it right now,” Ms. Eskamani said. “It just hasn’t been necessary.” One recent morning, Ms. Eskamani attended an hourlong phone conference with volunteers from...