The organizers of a mysterious network paid influencers to promote sexual smears of Vice President Kamala Harris as Democrats rallied around her to be their presidential candidate in late July. The attacks were part of a campaign that was, on the surface, an unremarkable part of a new wave of social media marketing. It’s blurred the lines in both US political parties between paid advertising and the posts of enthusiastic supporters — as partisan influencers often cast themselves — who...