When most people hear the phrase “one person, one vote,” they don’t stop to think about who counts as a person.

The U.S. Supreme Court gets to answer that in a case — Evenwel v. Abbott — that started here in Texas. The plaintiffs contend their votes don’t count as much as those of voters in other state Senate districts because the districts are designed to have the same number of humans in them, not the same number of voters.

It’s a simple idea, but changing who’s counted — the voters, instead of the humans — would wreck the country’s political maps, particularly in states like Texas where large numbers of people are not eligible to vote.

This is not only about drawing maps and which politicians win and lose in the next elections. It does not concern who is allowed to vote, but how their votes are distributed. It’s about representation — clout, if you’d rather — for the adults who are allowed to vote and for the felons and little kids and immigrants who live here but cannot.

One big problem is that there is no reliable count of eligible voters. The U.S. Census Bureau counts the population, but its numbers on eligible voters are merely estimates. At the first sign of a difference in opinion — and litigation is one of the few certainties when political maps are drawn — that federal agency would be on trial.

Changing who is counted changes who counts. If the measure is population and each district has to be about the same size, then each person theoretically has the same political power. Not all of them can vote, of course: Only adult citizens and non-felons can do that. But everyone is represented, at least on paper, and everyone who gets elected has some obligation to take care of everyone in his or her district.

You can argue that what makes sense on paper doesn’t make sense in real life. Not everybody votes, for one thing, and variable turnout in each district means some people get into office with fewer votes than others. Turnout varies so much that some people got into office with fewer supporters than candidates who lost races in other districts. It takes more than 50 percent of the voters to win, no matter how many or how few there are.

Even without turnout variables, not all districts are alike. Take congressional districts, for instance, since they are required to be as close to the same size in population as possible.

The 36 congressional districts in Texas each had 698,488 people in them when they were drawn. That seeming exactitude hides big differences. The 17th Congressional District, represented by Bill Flores, R-Bryan, has the same number of people in it as the 33rd, represented by Marc Veasey, D-Fort Worth. But Flores’ district has 532,324 adults — 62,868 more than Veasey’s. That means there are more children in the Veasey district than in the Flores district.

But the little ones are not the only ineligible voters. Veasey’s district also happens to have the smallest number of people who are allowed to vote — 305,120 — a number that excludes non-citizens as well as children. More than half of the people in his district are ineligible to vote.

On the other end of the scale, the biggest citizen voting-age population is represented by Lamar Smith, R-San Antonio; his 21st Congressional District has 489,820 such people.

Each district is the same size, but 70.1 percent of Smith’s residents can vote and only 43.7 percent of Veasey’s are eligible.

In a perfect election with 100 percent turnout among people eligible to vote, the voters in Veasey’s district would have proportionately more power. It would take fewer of them to put someone in Congress than it would take in Smith’s district. That said, each of the winners in those two districts would go to Washington, D.C., to represent the same number of people.

The plaintiffs in Evenwel v. Abbott live near Houston, in districts that are more like the areas represented by Flores and Smith. They contend in their lawsuit that lawmakers could have drawn districts with about the same populations and eligible voters at the same time, but instead packed them into districts in a manner that diluted their votes. They argued, too, that the U.S. Constitution protects voters as well as residents. A lower court of three judges — each appointed by a Republican president, George W. Bush — ruled against them.

The Supreme Court accepted the appeal last month; those judges will decide who counts, and for how much.