This...this can't be constitutional, let alone should not be allowed.
One of the privileges of being a citizen is having the right to vote. You can't just walk into the United States and vote.
This is ridiculous.
I can't think of any grounds on which it could be declared unconstitutional.
I did look back at the 15th Amendment. It does look like it's up to interpretation since it doesn't explicitly state that non-citizens can't vote. It just outlines how citizens can vote.
If I were a judge or justice, however, I would strike it down.