2 U.S.C. § 5503

In any action brought against any person for or on account of anything done by him while an officer of either House of Congress in the discharge of his official duty, in executing any order of such House, the United States attorney for the district within which the action is brought, on being thereto requested by the officer sued, shall enter an appearance in behalf of such officer; and all provisions of the eighth section of the Act of July 28, 1866, entitled “An Act to protect the revenue, and for other purposes”, and also all provisions of the sections of former Acts therein referred to, so far as the same relate to the removal of suits, the withholding of executions, and the paying of judgments against revenue or other officers of the United States, shall become applicable to such action and to all proceedings and matters whatsoever connected therewith, and the defense of such action shall thenceforth be conducted under the supervision and direction of the Attorney General.