The Supreme Court rejected a conservative challenge to the “one person, one vote” rule that counts nonvoters, such as children and prisoners, in drawing districts. Here’s what you need to know. (Victoria Walker/The Washington Post)

The Supreme Court unanimously ruled Monday that states may satisfy “one person, one vote” rules by drawing election districts based on the total population of a place, a defeat for conservative interests that wanted the districts based only on the number of people eligible to vote.

The case, Evenwel v. Abbott, was considered one of the most important on voting rights this term, and a decision the other way would have shifted political power away from urban areas, where Democrats usually dominate, and toward more Republican-friendly rural areas.

The court’s ruling left open the possibility that other methods of reapportionment might be constitutional. But the decision was clear that using anything other than total population would face certain Supreme Court review.

The leader of the effort to use voting-eligible population as the standard said it was unlikely that any jurisdiction would test such a system until after the 2020 census.

The Supreme Court has never explained what metric should be used to satisfy the “one person, one vote” standard that the court advanced in the 1960s, and Chief Justice John G. Roberts Jr. chose Justice Ruth Bader Ginsburg to write the majority decision.

“What constitutional history and our prior decisions strongly suggest, settled practice confirms,” Ginsburg wrote. “Adopting voter-eligible apportionment as constitutional command would upset a well-functioning approach to districting that all 50 States and countless local jurisdictions have followed for decades, even centuries.”

She added, “As the Framers of the Constitution and the Fourteenth Amendment comprehended, representatives serve all residents, not just those eligible or registered to vote.”

She was joined by Roberts and Justices Anthony M. Kennedy, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan. Justices Clarence Thomas and Samuel A. Alito Jr. agreed with the outcome but filed separate concurrences.

Currently, all states, with some minor variations, use total population for redistricting, and that standard is used to allocate congressional districts to the states after each census.

The general population contains millions of people who aren’t eligible to vote: children, legal and illegal immigrants, prisoners, and those who are disenfranchised. Except for prisoners, they are largely concentrated in urban areas.

Latino groups were especially cheered by the court’s ruling.

Rep. Linda T. Sánchez ­(D-Calif.), chairman of the Congressional Hispanic Caucus, said the challenge “was a conservative effort to undermine the growing influence of the Latino community. Today’s unanimous decision is a victory and reaffirms the fundamental principle that in the United States each person counts.”

The court’s decision was supported by all eight justices because it chose a middle ground between several competing positions. The challengers wanted only the ­voting-eligible population to be counted for redistricting. The Obama administration said that only total population would be constitutional. The state of Texas, whose state Senate districts were challenged, wanted the option to use either.

But the court stopped after deciding that the total population passed muster, declining to rule on whether any other method of drawing districts could be used. Perhaps because the court has shown itself equally divided in other cases after the death of Justice Antonin Scalia, Ginsburg wrote that anything more could wait.

The group that brought the challenge, the Project on Fair Representation, expressed disappointment that the justices “were unwilling to reestablish the original principle of one-person, one vote for the citizens of Texas and elsewhere.” The group’s plaintiffs had said that the unequal numbers of eligible voters in the Senate districts diluted the impact of their votes.

The project’s founder, Edward Blum, said: “The issue of voter equality in the United States is not going to go away. Some Supreme Court cases grow in importance over time, and Evenwel v. Abbott may likely be one of those cases.”

Blum’s tiny organization has brought challenges that questioned affirmative action in university admissions and led the court to strike a major portion of the Voting Rights Act.

Blum’s opponents said the court was simply recognizing what most people had always thought was the correct way to interpret court rulings demanding that states abandon redistricting practices that had kept power in rural white areas rather than letting it shift to rapidly growing urban areas.

Kathay Feng, director of redistricting for Common Cause, said in a statement that her organization “joined cities and counties across the country — from Los Angeles, CA to South Bend, IN to Atlanta, GA — to argue that everyone — young, old, city-dwellers and small town residents — deserves equal representation when it comes to providing police, fire, schools, and other services.”

Ginsburg struck a similar note. “Nonvoters have an important stake in many policy debates — children, their parents, even their grandparents, for example, have a stake in a strong public-education system — and in receiving constituent services, such as help navigating public-benefits bureaucracies,” she wrote.

“By ensuring that each representative is subject to requests and suggestions from the same number of constituents, total population apportionment promotes equitable and effective representation,” Ginsburg wrote.

Thomas and Alito departed from the majority as the opinion seemed to favor one method over another.

For starters, Thomas said, the court “has never provided a sound basis for the one-person, one-vote principle.” He said the court has become too involved in how states create districts.

The Constitution, Thomas wrote, “leaves States significant leeway in apportioning their own districts to equalize total population, to equalize eligible voters, or to promote any other principle consistent with a republican form of government.”

Alito also said the majority was reading too much into history. He said the issue was not completely settled by Monday’s decision.

“Whether a State is permitted to use some measure other than total population is an important and sensitive question that we can consider if and when we have before us a state districting plan that, unlike the current Texas plan, uses something other than total population as the basis for equalizing the size of districts,” Alito wrote.