The drosophila in the unguent
Of course, there's no way to prevent the briber from telling the bribee about a "contribution". Or, for that matter, taking out a billboard on the Beltway announcing the "contribution".
Amusing possibilities for fakeouts, though--"yeah, sure, really, I donated a million, trust me, it's in the pipeline; now gimme my legislation". A bit of doubt would put a little sand in the gears of the bribery machine, and every little bit helps.