My argument is simply that the citizenship question is clearly Constitutional. The fact that it was once on and was taken off or that various people don’t think it should be on doesn’t make it any less Constitutional. If other groups win elections and gain control of the Executive branch, they can add or take away questions.
There is zero legal support for your doctrine that says an act is unconstitutional if it was enacted with intent to benefit a political party. I agree any bill that has explicit language favoring any political group has to clear some Constitutional hurdles on equal protection under law. But if there is no direct violation of Constitutional provisions within the 4 corners, then it is Constitutional and good or bad motivation does not make it any less so.
What if many welfare state measures can be shown to be motivated by the intent to promote the Democrat Party. All unconstitutional then?