18 U.S.C. § 611 : US Code - Section 611: Voting by aliens

      (a) It shall be unlawful for any alien to vote in any election
    held solely or in part for the purpose of electing a candidate for
    the office of President, Vice President, Presidential elector,
    Member of the Senate, Member of the House of Representatives,
    Delegate from the District of Columbia, or Resident Commissioner,
    unless - 
        (1) the election is held partly for some other purpose;
        (2) aliens are authorized to vote for such other purpose under
      a State constitution or statute or a local ordinance; and
        (3) voting for such other purpose is conducted independently of
      voting for a candidate for such Federal offices, in such a manner
      that an alien has the opportunity to vote for such other purpose,
      but not an opportunity to vote for a candidate for any one or
      more of such Federal offices.

      (b) Any person who violates this section shall be fined under
    this title, imprisoned not more than one year, or both.
      (c) Subsection (a) does not apply to an alien if - 
        (1) each natural parent of the alien (or, in the case of an
      adopted alien, each adoptive parent of the alien) is or was a
      citizen (whether by birth or naturalization);
        (2) the alien permanently resided in the United States prior to
      attaining the age of 16; and
        (3) the alien reasonably believed at the time of voting in
      violation of such subsection that he or she was a citizen of the
      United States.