Bribery is illegal. Even of the form often presupposed on HN threads: "Congressman, if you vote for this bill, Exxon will make sure you have hundreds of donors contributing thousands each to your next campaign." I'm 100% sure such quid pro quos still happen, but because they're unambiguously against the law, they represent at most a tiny fraction of the lobbying dollars that flow through DC.

Most lobbying is boring stuff like proposed amendment drafting, outreach to Congressional and agency staff, highly targeted issue & message test polling, ...