18 U.S.C. § 879 : US Code - Section 879: Threats against former Presidents and certain other persons

      (a) Whoever knowingly and willfully threatens to kill, kidnap, or
    inflict bodily harm upon - 
        (1) a former President or a member of the immediate family of a
      former President;
        (2) a member of the immediate family of the President, the
      President-elect, the Vice President, or the Vice President-elect;
        (3) a major candidate for the office of President or Vice
      President, or a member of the immediate family of such candidate;
      or
        (4) a person protected by the Secret Service under section
      3056(a)(6);

    shall be fined under this title or imprisoned not more than 5
    years, or both.
      (b) As used in this section - 
        (1) the term "immediate family" means - 
          (A) with respect to subsection (a)(1) of this section, the
        wife of a former President during his lifetime, the widow of a
        former President until her death or remarriage, and minor
        children of a former President until they reach sixteen years
        of age; and
          (B) with respect to subsection (a)(2) and (a)(3) of this
        section, a person to whom the President, President-elect, Vice
        President, Vice President-elect, or major candidate for the
        office of President or Vice President - 
            (i) is related by blood, marriage, or adoption; or
            (ii) stands in loco parentis;

        (2) the term "major candidate for the office of President or
      Vice President" means a candidate referred to in subsection
      (a)(7) of section 3056 of this title; and
        (3) the terms "President-elect" and "Vice President-elect" have
      the meanings given those terms in section 871(b) of this title.