Since the 1960s, the high court has enforced the "one person, one vote" rule to require that election districts be roughly equal in population. This rule is based on the equal protection clause of the 14th Amendment. and it is applied to all election districts, whether for members of Congress, state legislators, county supervisors or local school boards.
At least once a decade, these districts may be redrawn based on new census data. The U.S. Census Bureau seeks to count the total population, including noncitizens and immigrants in the country illegally.
In its appeal, the Project on Fair Representation based in Austin, Texas, says the districts should be balanced based on the number of eligible voters. The group sued on behalf of Sue Evenwel, a leader of the Texas Republican Party. She lives in Titus County in east Texas, where her state Senate district had 533,010 citizens of voting age in 2011. However, another Senate district had 372,00 citizens of voting age.
The appeal in Evenwel vs. Abbott argues that her right to an equal vote is being denied because Texas officials relied on the census data to balance the districts. Requiring states to switch to counting only citizens will "ensure that voters are afforded the basic right to an equal vote," her lawyers said.
Forcing states like California and Texas to rewrite districts based on citizens versus residents brings up all sorts of second-order shenanigans, the one off the top of my head is again, forcing these states to identify the undocumented rather than relying on estimates and then deporting them en masse. There's a lot of messy things that could result from a SCOTUS order along those lines.
We'll see where this goes, but this seems on first pass like yet another GOP attempt to harm minority voters through the Well Actually process.