Law Professor Rick Pildes has this article at Scotusblog about the upcoming U.S. Supreme Court hearing in Evenwel v Abbott, 14-940. The Court will hear this case late in 2015 or early in 2016. Texas, like almost all states, bases redistricting on population, not the number of voters. States are free to decide which standard to use.
The Texas individuals who brought the case believe that the U.S. Constitution requires that redistricting create districts of equal numbers of voters, not equal numbers of residents. Pildes thinks it is somewhat likely that the lawsuit will boomerang on the people who brought the lawsuit, and that the outcome might be that the Supreme Court says districting must be based on residents, not number of voters. Of course the Court is also free to maintain the status quo, in which any state can choose either method. Thanks to Election Law Blog for the link.