It already IS a felony!
Here’s the law:
18 USC 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.
(Added Pub. L. 104–208, div. C, title II, §216(a), Sept. 30, 1996, 110 Stat. 3009–572 ; amended Pub. L. 106–395, title II, §201(d)(1), Oct. 30, 2000, 114 Stat. 1635 .)
Article on it:
Most discoveries of non-citizens on the registration rolls are therefore accidental. Though the Department of Justice has no procedures in place for a systematic investigation of these types of criminal violations, in just a three year period, it prosecuted and convicted more than a dozen non-citizens who registered and voted in federal elections in Alaska, Florida, the District of Columbia, and Colorado. [32] Among them was an alien in southern Florida, Rafael Velasquez, who not only voted, but even ran for the state legislature.[33] Eight of the 19 September 11 hijackers were registered to vote in either Virginia or Florida-registrations that were probably obtained when they applied for driver’s licenses.[34]
http://www.heritage.org/research/reports/2008/07/the-threat-of-non-citizen-voting