Each decennial census which takes place more than one year after the ratification of this amendment shall count the number of citizens who are either under the age of eighteen or eligible to vote for the most populous branch of the legislature of their State.
This count shall replace the popular count as the basis for the calling of an Amendment Convention by the States, representation within an Amendment Convention, the apportionment of Representatives among the States, the creation of representative districts within the States, and the number of votes per Senator.
Just as the first principle of democratic societies is equal voting power over the law, the first purpose of the census is apportionment. Non-citizens should not be counted for the purposes of apportionment. They currently do count, which warps the voting power of citizens who reside in the same district or state. The same goes for felons and ex-felons who are disfranchised by their state.
The second, more modern purpose of the census is distribution of federal spending. Since spending is ultimately controlled by Congress, Congress should decide how to handle this purpose of the census as well. Congress would likely decide that the census should continue to count non-citizens, perhaps with an additional question about citizenship status, and perhaps with statutory text such as, “Personal information collected by the census may not be used against any individual by any government agency or court.” Congress could conceivably decide to exclude non-citizens from the census and to estimate total population by sampling or other means.
The census and apportionment relationship is a first-principle issue best left to the Constitution.
The distribution of spending as it relates to non-citizens is a second-principle issue best left to Congress.