Every person who obtains, or seeks to obtain, money or other thing of value from another person upon a pretense, claim, or representation that he can or will improperly influence in any manner the action of any member of a legislative body in regard to any vote or legislative matter, is guilty of a felony. Upon the trial no person otherwise competent as a witness may be excused from testifying concerning the offense charged on the grounds that the testimony may criminate himself, or subject him to public infamy. The testimony shall not afterwards be used against him in any judicial proceeding except for perjury in giving the testimony.