By Ariane de Vogue

Washington (CNN) – The Supreme Court handed conservative challengers a loss Monday in a key voting rights case.

In a unanimous result, the court said a state can draw legislative districts based on total population. At issue in the case was the “one person, one vote” principle dating back to the 1960s, when the court held that state legislative districts must be drawn so they are equal in population.

But, until Monday, justices never specified whether that doctrine applied to the general population or to the voting population. All states currently draw lines based on general population, but two conservative plaintiffs from Texas argued their vote was being diluted in relation to other districts that had the same number of people but fewer voters.

The Obama administration and state of Texas opposed the lawsuit. Civil rights groups watched the case carefully, fearful that if the court were to rule with the plaintiffs, it could potentially shift power from urban areas — districts that tend to include a higher percentage of individuals not eligible to vote such as non-citizens, released felons and children — to rural areas that are more likely to favor Republicans.

Justice Ruth Bader Ginsburg wrote the opinion. “History, our decisions, and settled practice in all 50 States and countless local jurisdictions point in the same direction: Total-population apportionment meets the Equal Protection demand, by rendering each representative alert to the interest and constituent-service requests of all who dwell in the representative’s district,” she wrote.

“The rule appellants urge has no mooring in the Equal Protection Clause. The Texas Senate map, we therefore conclude, complies with the requirements of the one-person, one-vote principle,” the opinion adds. “Because history, precedent, and practice suffice to reveal the infirmity of appellants’ claims, we need not and do not resolve whether, as Texas now argues, States may draw districts to equalize voter-eligible population rather than total population.”

Edward Blum, the director of a conservative group called Project on Fair Representation, backed the challenge by Texas residents Sue Evenwel and Edward Pfenninger. Blum’s group was also behind a 2013 case that invalidated a central provision of the Voting Rights Act as well as a case this term seeking to strike down a race-conscious admissions program at the University of Texas.

Blum said Monday his group is “disappointed” in the ruling. “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,” he said in a statement.

Other options available?

In ruling that states may draw legislative districts based on total population, the court declined to answer whether those states could look to voter population as a metric.

“The justices left open the possibility that states could use other metrics, like voter population, for drawing district lines; all they held today was that states don’t have to use those metrics, and that the Constitution is not offended by district lines based upon total population,” said Steve Vladeck, professor of law at American University and CNN legal analyst.

Justices Samuel Alito and Clarence Thomas agreed with the court’s bottom line Monday, but wrote separately.

Alito said he agreed with the court that “Texas’ use of total population did not violate the one-person, one-vote rule,” but he said that in another case the Court could go take another look at the use of voting population. “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,” he said.

“This was a sweeping rejection of a politically motivated lawsuit, with not even a single Justice voting for the result urged by the challengers,” said Elizabeth Wydra, president of the Constitutional Accountability Center, a group that filed a brief in support of Texas.

“While Justice Ginsburg’s opinion didn’t reach the question of whether states can ever lawfully draw districts based only on voter-eligible population, it is difficult to see how such a claim could survive her powerful recounting of how our Constitution’s drafters –both at the founding and during Reconstruction — clearly envisioned total population as the ‘theory of the Constitution’ when it comes to representation.”

The plaintiffs argued that their vote was being diluted in relation to voters in other districts and that Texas must look primarily at the total number of eligible voters when it draws district lines.

“This appeal presents a fundamental question,” William Consovoy, a lawyer for Evenwel, told the justices during oral arguments in December. “That question is whether the one person, one vote rule affords eligible voters any reasonable protection.”

Civil rights groups feared that Latino communities in certain states with nonvoting residents, as well as children and others, would be sharply disadvantaged if the court were to side with Evenwel. “Drawing districts to equalize people is the only way to ensure that the communities where people live and work are fairly represented in the nation’s legislatures,” Michael Li, counsel for the Brennan Center’s Democracy Program said after oral arguments.

Ginsburg addressed that in her opinion.

“Adopting voter-eligible apportionment as constitutional command would upset a well-functioning approach to districting that all 50 States” have followed for “decades, even centuries,” she wrote.

She added, “Non voters 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 said.

Also supporting Texas was Nathaniel Persily of Stanford Law School, who said that if the court were to say that the Constitution requires states to use the voting population, it could unleash a series of questions regarding the reliability of voter lists and surveys. “A national database of eligible voters does not exist and will not exist in the foreseeable future,” he said in an amicus brief.