They saw what you did there, Texas.
Since the 1960s — the case of Wesberry v. Sanders – to be specific — we have had a “one person, one vote” which says that state legislative districts must be drawn so they are equal in population.
But the Republican asshole legislature in Texas (of course) wanted legislative lines to be drawn based on eligible or registered voters instead of total population as measured by the US Census Bureau, thus not counting children, immigrants (documented and undocumented), prisoners, and other nonvoters. They claimed the current system, by including nonvoters, denies “eligible voters their fundamental right to an equal vote.”
Which is bullshit. What they were really trying to do was a transparent attempt to dilute the vote of minorities in urban areas by not counting people, but counting registered voters instead. Combined with conservative purges of voter roles, this would shift power to more rural areas. In other words, while the population of a city might have 1 million people, if only 200k were voters, the district would be apportioned to the voter rolls. This would shift power to more rural districts, and thus give conservatives an advantage, much like how the Senate inflates power in the hands low population states.
SCOTUS saw through this. The Notorious RGB wrote the opinion, which was — believe it or not — UNANIMOUS.