18 U.S.C. § 431: Contracts by Member of Congress

Whoever, being a Member of or Delegate to Congress, or a Resident Commissioner, either before or after he has qualified, directly or indirectly, himself, or by any other person in trust for him, or for his use or benefit, or on his account, undertakes, executes, holds, or enjoys, in whole or in part, any contract or agreement, made or entered into in behalf of the United States or any agency thereof, by any officer or person authorized to make contracts on its behalf, shall be fined under this title. All contracts or agreements made in violation of this section shall be void; and whenever any sum of money is advanced by the United States or any agency thereof, in consideration of any such contract or agreement, it shall forthwith be repaid; and in case of failure or refusal to repay the same when demanded by the proper officer of the department or agency under whose authority such contract or agreement shall have been made or entered into, suit shall at once be brought against the person so failing or refusing and his sureties for the recovery of the money so advanced.