There are many types of outside groups that now sponsor ads in federal elections. Of particular interest to many are the presence of mandates for some groups to disclose their donors to either the IRS (for 527s) or Federal Election Commission (for PACs and super PACs). 501c4 non-profit groups do not have to disclose publicly their donors; as such they are often referred to as “dark money” groups. Previous work has demonstrated that dark money is more prevalent early in election cycles and that ads aired closer to elections are more likely to be from full disclosure groups. See “Outside Group Activity, 2000–2016,” Special Report from Wesleyan Media Project, August 24, 2016: http://mediaproject.wesleyan.edu/releases/disclosure-report/.