The U.S. Supreme Court on Monday unanimously upheld Texas' method of outlining voting districts, rejecting a conservative argument that could have shifted significant voting power to predominately white, rural areas.

The court found in Evenwell v. Abbott (pdf) that Texas did not violate the long-established "one person, one vote" doctrine in carving out its voter maps. The conservative activists who brought the case argued that the state should only consider eligible voters, rather than total populations—an argument the court rejected.

Monday's decision allows, but does not require, all states to count every resident when mapping out legislative districts after each census.

Legislators "serve all residents, not just those eligible or registered to vote," Justice Ruth Bader Ginsburg wrote for the majority. Non-voters, including children, "have an important stake in many policy debates," like education.

Democracy advocates lauded the ruling.

"Today's decision affirms one of our most fundamental values as Americans: that every person counts," said Common Cause president Miles Rappaport. "Our nation's charter begins with the words 'We the People;' that means everyone—not just those who vote—is entitled to equal representation at every level of government."

Wade Henderson, president of the Leadership Conference on Civil and Human Rights, added, "Today's unanimous ruling strongly affirms our nation's longstanding constitutional principle that everyone deserves representation in our democracy. We applaud the Supreme Court for rejecting the challengers' misguided, politically-motivated attempt to pervert this principle and exclude countless children and other non-voters from political representation."

An alternative ruling in the case could have taken significant legislative influence away from highly populated urban centers, which tend to be more racially diverse and Democratic, and handed it to rural areas with lower populations, a higher percentage of white voters, and more Republican leanings.

That also could have disenfranchised non-voters like immigrants and convicted criminals, the court noted.

Nina Perales, vice president of litigation with the Mexican-American Legal Defense and Education Fund, told Reuters on Monday that the ruling was a "clear and important victory."

It "protects the rights of all people across the U.S. to be represented by their officials and be counted when electoral maps are drawn," she said.