It's not typical for the Supreme Court circa 2016 to issue unanimous opinions on politically charged cases — especially when those cases are about voting rights.

But on Monday, in the case Evenwel v. Abbott, the court ruled 8-0 against a proposal to change who got counted in drawing up congressional districts — which would have given more political power to largely white areas while reducing the power of heavily Hispanic and Asian-American ones.

"One person, one vote" versus "one voter, one vote"

Right now — and, under today's Supreme Court decision, for the foreseeable future as well — states carve up congressional districts to have a roughly equal number of people in each district, a principle known as "one person, one vote." There's some variation allowed in how many people a given district can have, but the general principle is that each member of the House of Representatives (and of state legislatures) is responsible for representing and looking out for the population that lives in his or her district.

But the term "one person, one vote" is unhelpfully confusing, because some of the people who get counted in drawing up congressional districts can't vote: They're noncitizens, or children, or felons who've lost voting rights under their state laws. In some districts in southern Texas, for example — as this Pew Research Center analysis shows — just a little over half of all residents are eligible to vote.

To the Texans who brought the Evenwel v. Abbott case, that's exactly the problem.

If districts are drawn on equal population but have different numbers of eligible voters, a voter in a central Texas district with 525,000 eligible voters has, essentially, 0.0000019047619 of a voice in selecting his representative, while a voter in a southern Texas district with 386,000 eligible voters has 0.00000259067358 of a voice. The southern Texas voter's vote "counts" about 33 percent more.

So Sue Evenwel (and her co-plaintiff) sued the state of Texas, complaining that the current system is unconstitutional. They proposed a plan to make each vote count an equal amount — call it "one voter, one vote" — by drawing congressional districts to represent roughly equal numbers of eligible voters.

In other words, the map would be redrawn so that more congressional districts were carved from the parts of the state with more voters (central and northern Texas) while parts of the state with more voting-ineligible residents (like southern Texas) would be grouped into fewer, larger districts.

Even though Evenwel started off with a suit against the state of Texas, by the time the case got to the Supreme Court, Texas was sort of on her side. The Texas government took the position that it was okay to use total population, but that it also would have been okay to use a different measure of population — like the number of people in a district who were citizens of voting age.

The Evenwel plan would have given less political power to Hispanic and Asian communities

On one level, the Supreme Court case was a philosophical question: Does a member of Congress or a state legislature represent the whole population of his or her district, or does he or she represent the voters within the district? The Supreme Court, in a decision written by Justice Ruth Bader Ginsburg, came down firmly on the side of the former: "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."

But the reason that the case was so important — and why it's so surprising the Court's ruling was unanimous — is that it's also a debate about the relationship between demographics and political power.

After all, some groups are more likely to be eligible to vote than others. Nearly 80 percent of white people in America are citizens over the age of 18; fewer than half of Hispanics in America fit that description.

In practice, then, carving up congressional districts based on eligible voters would result in fewer representatives for heavily Hispanic and Asian parts of the US. And, unsurprisingly, the seats it would have gotten rid of would have mostly been Democratic.

Voting rights groups were seriously concerned about what would happen if the Supreme Court told states to ignore people who weren't eligible to vote when drawing up districts. When the court heard oral arguments in December, the Southern Poverty Law Center called the case "a frontal assault on the core of the 14th Amendment, which was enacted after the Civil War to give equal representation and protection to all people."

Luckily for those groups, the Court ultimately rejected the Evenwel plan soundly: "History, precedent, and practice suffice to reveal the infirmity of appellants' claims," Ginsburg wrote. And it did so unanimously.

But the Court didn't settle the question entirely, and instead set up what's likely to be the next battle: the Texas government's proposal to use citizen voting age population, rather than total population, to draw up districts. That sort of plan would have similar effects to the Evenwel plan. But two conservative justices — Clarence Thomas and Samuel Alito — both wrote concurring opinions agreeing that it wasn't constitutional to force a state to ignore population, but signaling that it would be okay if a state decided to count population in a different way (i.e., by only counting adult citizens).

It's not clear whether there are more than two votes on the Court to get rid of the standard that every person counts in districting. Chief Justice John Roberts and Justice Anthony Kennedy joined the Court's liberals in signing onto Justice Ginsburg's opinion, which didn't address Texas's proposal one way or the other. But it's fair to bet that the fight over what "one person, one vote" actually means will continue.

Gerrymandering, explained