Conservatives have a different view. In 1964, the Supreme Court’s historic one-person-one-vote ruling ordered Alabama to draw legislative districts with equal populations, arguing that anything else made some people’s votes count less than their neighbors’. But including noncitizens in the calculus upends that, conservatives argue: Given two districts with equal populations, the votes in a district with many noncitizens and fewer citizens will count more than a district composed entirely of citizens with voting rights.

“When 600,000 citizens can elect one representative to a legislative body, whereas their neighbor next door requires 750,000 citizens to elect one representative, this is in my opinion — and I think in the opinion of others — intrinsically unfair,” said Ed Blum, the director of the conservative legal advocacy group Project on Fair Representation.

Mr. Blum took the issue to the Supreme Court in a 2016 case that sought to base Texas legislative maps on the number of voting-age citizens instead of on total population. The justices ruled that the formula that included noncitizens was constitutional.

But they pointedly left the door open to other metrics. If the census provides a way to count noncitizens, Mr. Blum said, the issue is likely to return to the courts quickly.

Legal and historical precedents do not provide clear guidance as to who is right. On the one hand, the Constitution clearly requires the House of Representatives to be apportioned based on “the whole number of persons,” citizens and otherwise. And by counting slaves as three-fifths of a person, the founders specifically accorded representation to people who had no political rights. The same is true of children, who are politically powerless yet universally counted for redistricting and apportionment.

But long before the 1964 one-person-one-vote decision, states apportioned and drew legislative districts in a host of ways, including by counting only citizens. New York counted citizens in its own census for purposes of reapportioning its State Legislature until 1970, when it abandoned the practice. Even now, both Kansas and Hawaii do not count university students or families at military installations unless they already are state residents — even though those people are not counted elsewhere in the decennial census.

In 1966, the Supreme Court blessed Hawaii’s rule, suggesting that a legislature could set any standard as long as it led to an apportionment scheme that was not “substantially different” from one that was clearly permissible.