Non-citizens can’t vote, of course, but their numbers are traditionally counted in the census. The more people in an area, even if they’re not American citizens, the more that area gains representation in government. If non-citizens are too frightened to answer their census questionnaire, then urban areas (where the undocumented tend to live) lose influence.
The fear is that this will shift power to rural areas. Indeed, it seems to be part of a larger plan on the part of the GOP to make certain that only their supporters can vote.
Not a pleasant thought, given the kind of man those voters put into the White House.