I refer you to the Constitution of the United States.
Amendment XV, Section 1:
The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude….
Amendment XIX:
The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of sex.
Amendment XXIV, Section 1:
The right of citizens of the United States to vote in any primary or other election for President or Vice President, for electors for President or Vice President, or for Senator or Representative in Congress, shall not be denied or abridged by the United States or any State by reason of failure to pay any poll tax or other tax.
Amendment XXVI, Section 1:
The right of citizens of the United States, who are eighteen years of age or older, to vote shall not be denied or abridged by the United States or by any State on account of age.
Clearly, the Constitution contemplates that only citizens of the United States may vote. It might be argued that the Constitution applies only to elections for federal office. If that were the case, States and localities would have to require voters to show proof of U.S. citizenship and give them ballots that pertain only to State and local offices and issues.
But I would go further than that. Allowing non-citizens to vote in any election in the United States violates Section 1 of Amendment XIV:
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws [emphasis added].
If some localities allow non-citizens to vote, but others in the same State disallow non-citizen voting, citizen-residents of localities in the second category are denied the equal protection of the laws (i.e., the power of their votes is diluted).
Further, the same logic applies across States. Those States that allow non-citizens to vote for federal offices are giving them “privileges” that are denied to citizens of States that allow only citizens to vote.
Politics and Prosperity 