I don’t have a problem with this ruling. It’s a proper read of the Constitution. There’s nothing in the census language that requires citizenship. Scalia would have probably decided the case the same way.
US Congress, yes.
State legislature/State Senate, no.
It would seem to be “denying to its citizens equal protection of the laws.”
The question is, “MAY a state apportion legislators on the basis of eligible voters, or MUST they apportion on population?”
One wrinkle is that the Census Bureau does not ask respondents whether they are legal. Any state wishing to apportion that way would have to conduct its own census.
However, school districts conduct their own censuses and seem to find the money.