If a deal is struck and consummated, both parties are liable.

If a deal is struck but not consummated, then potentially only the party who actually did a material act is liable. That could be the special interest who made the donation hoping for a particular official act that never materialized, or an official act that was done with the understanding of a later reward that ends up not happening (such as a job or consulting payments once they retire from public office).

If a person brags about how they bribe public officials, I say take them at their word.