I saw this question about what states that don't use voter ID do to ensure non-citizens don't vote, and the answer seems to be little, if anything. Given the current political climate (where some states want to be "sanctuaries" for non-citizens), it's not hard to see a state like California (which is openly defiant on the issue) looking for ways to take the issue even further.

Let's say California decides to go further and passes a law that openly allows non-citizen residents to vote in California elections (i.e. they move beyond mere non-enforcement to suborning it). In the current political climate there would be open angst at such a move, but what could be done from a practical standpoint about the issue? It appears to run afoul of Federal law, but it's not clear how you would enforce it in a state where they ignore some Federal laws already.