Route 1
Anonymous Donors→Non-profit Organization (501c4)→Issue Ads
Registered as a social welfare organization, there is no requirement to disclose donors. While nominally for issue ads, most are used to attack or support specific candidates.
Route 2
Non-profit Organization→Super PAC→Election Ads
Super PACs disclose their own donors, but since the donors (non-profits) are not required to disclose their own, the original source remains hidden.
Route 3
3 Weeks Before Election→Establishment of Pop-up Super PAC→Post-election Disclosure
Super PACs established after the disclosure deadline are only required to disclose donors after the election is over.