Should that change, it would mean that children, and immigrants, both legal and illegal, and other groups would be excluded from the population count when state legislative districts are drawn.
The lawsuit before the Supreme Court seeks to do just that.
Two Texas voters are challenging the 2010 Texas reapportionment as unconstitutional because it created 31 state Senate districts to be as equal as possible in total population. The challengers claim that the correct constitutional metric should be eligible voters, not total population. Using that standard, they say that the Texas Senate map gives them roughly 50 percent less voting power.
Sue Evenwel, a member of the Texas State Republican Executive Committee, is the lead plaintiff. She notes, for instance, that her mainly rural district has 584,000 citizens eligible to vote, while a neighboring rural district, equal in total population, has only 372,000 eligible voters. Evenwel argues that the Texas districting scheme violates the "one person, one vote" principle because her vote has less weight relative to a person in a neighboring district with fewer eligible voters.
The Republican-dominated Texas government nonetheless defends total population as the best and most reliable means of ensuring equality in representation.
"The federal census data right now doesn't ask for citizenship," notes Texas Solicitor General Scott Keller. "So, we are using the best data set available."
Keller maintains that under previous Supreme Court rulings, the state can choose which population it is seeking to equalize, as long as it is not targeting one group for discrimination. "Texas, like every other state, has used total population for decades," he observes, adding that "there has been no claim that there's been unfair representation or our system of democracy has been undermined."
The challenge to the Texas redistricting has been coordinated by conservative activist Edward Blum. Some have called him the "mastermind" behind a series of cases that have successfully challenged long-established civil rights principles. Two years ago, Blum and a small cadre of lawyers succeeded in gutting a key portion of the 1965 Voting Rights Act. This week he has two more cases before the high court: the legislative districting case and another case he spearheaded, also from Texas, challenging affirmative action programs in state college admissions.
Blum used to be quite open about his work, but he is no longer talking to broadcasters, he says, because he isn't good at it. The lawyers arguing the cases are not talking either. So we turned to Ilya Shapiro, a senior fellow at the conservative and libertarian Cato Institute.
"If you have a situation where one person's vote is effectively worth half of what another person's vote is in the neighboring district, that, I think, is a problem," Shapiro says.
Shapiro points to Utah, a state with a high birthrate, as an example of a state that has greater representation for rural districts with lots of children, than for the urban and "more hipster" district that includes the city of Salt Lake. But ultimately, he tracks the disparities between total population and eligible voters to the immigrant population.
In the last half century, he says, total population has generally tracked eligible voting population. The late 1960s and early 1970s represented the "lowest foreign-born population in this country that we've had in at least the last hundred years or so," Shapiro adds. "And now, we're approaching the highest again, so that's why you have this disparity."
Shapiro, who wrote a brief siding with the challengers in this case, contends that the Constitution is based on voter equality and not population equality.
But David Gans, of the liberal-leaning Constitutional Accountability Center, co-authored a contrary brief, noting that when Congress debated and passed the 14th Amendment following the Civil War, the fight was over who should be covered by the equal protection guarantee.
On one side were those making the same argument that the challengers are in this case, he says, namely that "representation should be based on number of voters." Had that view prevailed, he notes, it "would have excluded women; it would have excluded aliens; it would have excluded children."
But, he continues, that view did not prevail; it was repeatedly voted down by the House and Senate.
The theory that the framers of the 14th Amendment adopted was to include everyone in the population so as to ensure "that everyone's voices are accounted for in representation," he says.
There is one other aspect of this case: the pragmatic difficulty of creating districts of equal numbers of voters when those numbers are not compiled anywhere, when voting rolls are notoriously inaccurate, fluctuating and unpurged, and when people age into the voting population every day.