With respect to the public office holders. Any "knowledge" of a bribe either offered, accepted or declined, that is not reported falls under misprision of treason.
In the United States, misprision of treason is a federal offense, committed where someone who has knowledge of the commission of any treason against the United States, conceals such knowledge and does not inform the President, a federal judge or State Governor or State judge 18 U.S.C. § 2382.
The circumstances that make bribery treasonous in fact is found under the High Crimes and Misdemeanor phrase in the Constitution at Art. II Sec. 4, "Treason, Bribery, or other high Crimes and Misdemeanors".
The Constitution sets specific grounds for impeachment. They are “treason, bribery, and other high crimes and misdemeanors.” To be impeached and removed from office, the House and Senate must find that the official committed one of these acts.
The Constitution defines treason in Article 3, Section 3, Clause 1:
Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.
The Constitution does not define bribery. It is a crime that has long existed in English and American common law. It takes place when a person gives an official money, gifts, a privilege or benefit (all emoluments), that may tend to influence the official’s behavior in office.
emolument (n.) mid-15c., from Old French émolument "advantage, gain, benefit; income, revenue" (13c.) and directly from Latin emolumentum "profit, gain, advantage, benefit,"...
Emoluments Clause. "No Title of Nobility shall be granted by the United States: And no Person holding any Office of Profit or Trust under them, shall, without the Consent of the Congress, accept of any present, Emolument, Office, or Title, of any kind whatever, from any King, Prince, or foreign State."
The Title of Nobility Clause is a provision in Article I, Section 9, Clause 8 of the United States Constitution, that prohibits the federal government from granting titles of nobility, and restricts members of the government from receiving gifts, emoluments, offices or titles from foreign states without the consent of the United States Congress. Also known as the Emoluments Clause, it was designed to shield the republican character of the United States against so-called "corrupting foreign influences". This shield is reinforced by the corresponding prohibition on state titles of nobility in Article I, Section 104, and more generally by the Republican Guarantee Clause in Article IV, Section 45.
Unfortunately lobbying gets a customary pass even though it lawfully qualifies as an emolument.