Who paid for it
Independent expenditures are reported to the FEC by the committees that make them. The contributions funding those committees are reported separately. This site joins the two, so spending can be read back to the individuals who paid for it.
Donor-level figures may be summed because they describe the donor. Candidate-level figures may not, and are never summed anywhere on this site. Why.
Selection
Donors are selected mechanically: top ten by attributed spend among donors whose attributed spend leans Republican, top ten among those leaning Democratic, both from the same query. Lean is derived from the support and oppose data, never assigned. Inclusion thresholds are not yet set; until they are, this index lists every donor configured in the repository.