In a unanimous ruling on Monday, the U.S. Supreme Court rejected a challenge to the way Texas -- and every other state -- draws its legislative districts.

Following the principle of "one person, one vote," states draw legislative and congressional districts that are equal in population. The case, Evenwel vs. Abbott, addressed the question of whether states should use only the voter-eligible population or the total population.

The court ruled that states can continue to use the total population. Six of the eight justices suggested that the question remains open as to whether states should be required to use total population. In concurring opinions, justices Clarence Thomas and Samuel Allito agreed that Texas couldn't be forced to use eligible voters as the basis of its count but said any state could do so.

The ruling maintains the power of places -- most of which are Democratic -- that have large numbers of residents who can't or don't register to vote. That includes immigrants who aren't citizens, children and prisoners.

Had the court ruled the other way, the decision would have been a boon for Republicans because rural areas, which are typically more conservative, would have gained strength at the expense of urban areas. Such a ruling would also have created an administrative nightmare for states, which rely on Census data in calculating total population. It wasn't clear how states would have determined the population of eligible voters.

The plaintiffs in this case were Sue Evenwel and Edward Pfenninger, voters who live in a pair of Texas state Senate districts with high percentages of eligible and registered voters. They argued that redistricting based on total population diluted the value of their votes, relative to those cast in other districts.

The 14th Amendment, which contains the Equal Protection clause that is the basis of redistricting law, refers to the rights of "persons," not "citizens."

"'One person, one vote' is a convenient shorthand but a misleading one," said Michael McDonald, a voting expert at the University of Florida. "There is no state that has chosen an alternative" to total population, although a few make adjustments to Census data.

Evenwel and Pfenninger were represented by the Project on Fair Representation, a conservative group.

"We are disappointed that the justices were unwilling to re-establish the original principle of one person, one vote for the citizens of Texas and elsewhere," said Edward Blum, the group's president. "The issue of voter equality in the United States is not going to go away."