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Some people think it’s unfair to have more eligible voters in one legislative district than in another — that basing things solely on total population is the wrong way to draw political maps. But that’s only one way the lines might be seen to slight a particular group of Texans.

The question stems from a lawsuit that went to the U.S. Supreme Court this week challenging the current maps for Texas Senate elections. The plaintiffs argue that those maps — drawn to put approximately the same number of people in every district — put them at a disadvantage by including unequal numbers of eligible voters in each district.

Those districts were drawn according to total population instead of the number of people who are eligible to vote. In the Evenwel v. Abbott case argued this week, two Texas voters said their votes are diluted because Texas Senate districts have the same populations but not the same numbers of voters. Each voter has a louder voice in districts with relatively fewer voters.

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But the number of eligible voters in each district is far from the only difference that might matter to Texans, whether they vote or not.

Age: Children can’t vote. Worse, they are unevenly distributed. In 31 Senate districts based on total population, the number of children in the districts ranged in 2014 from a low of 191,349 in Senate District 28 in West Texas to a high of 262,826 in Senate District 27 in South Texas. It’s hard to base maps on this number because the people who start each decade at age 9 become eligible to vote before the maps are redrawn.

(All of these numbers, by the way, are calculated by the Texas Legislative Council using estimates from the U.S. Census Bureau’s American Community Survey. Unlike the decennial census, which counts every person in the country, the ACS numbers are estimates of changes that occur between each census.)

Registered voters: The number of registered voters per district varies from a low of 295,657 in Houston’s Senate District 6 to a high of 578,484 in the fast-growing Senate District 25 in the Hill Country between Austin and San Antonio. Voter registration numbers are insecure and inaccurate; maps based on these would be easy for political operatives to manipulate.

Turnout: The number of people who vote changes with every election. Some elections are more consequential or more competitive, and the variance by district is tremendous. In the last presidential election, in 2012, 138,163 people voted in Houston’s SD-6, while 366,670 voted in SD-25 in the Hill Country. Look at that another way: Sylvia Garcia could have won her Senate seat with 69,082 votes, while Donna Campbell needed a minimum of 183,336 votes to win hers.

Party: Redistricting is all about party, about politicians picking their voters, about a legislative majority grabbing the highest possible number of seats. And they are very good at it. The Republican majority in Texas drew 20 reliably Republican districts and 11 reliably Democratic ones.

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If you’re in the minority party in your part of Texas — whether you are a voter or a candidate — you don’t stand a chance of putting your favorite in office unless the other side makes a bone-headed mistake.

The closest districts are not very close, and the most partisan districts are off the scale: In Senate District 31, which includes the Panhandle and the Permian Basin, the average Republican beat the average Democrat in a statewide race by 58 percentage points in 2012.

On the other hand, the average statewide Democrat won in Houston’s Senate District 13 by 67.4 percentage points. The average statewide advantage for the Republicans was just under 15 percentage points. 

Racial and ethnic minorities: Redistricting is probably the most litigious area of election law, and the rights of minorities are at the core of most of the fights. The federal Voting Rights Act is supposed to protect historically disadvantaged minorities from discrimination at the voting booth. The upshot is that a more or less proportional number of legislative districts have to be drawn so that minority voters have a reasonable chance of electing their candidates of choice.

By design, the mix of race and ethnicity in the districts is imbalanced. If that were not the case, Texas would have fewer Hispanic and black lawmakers — and more Republicans than it has now.

Geography: While Galveston and Amarillo have some common interests, like taxes, windstorm insurance and public education, they are as different as shrimp and steers. Communities of interest are important in redistricting but lose out to other factors, like politics, voting rights and population. Rural Texas used to have more representation than it does now. And some districts are strikingly strange. Midland and Odessa are connected to Amarillo, but not to Lubbock, which sits in between. State Sen. Judith Zaffirini, D-Laredo, represents three border counties, but also part of Austin.  The senator who represents eastern Collin County is Craig Estes, a Republican from Wichita Falls.

Even population: The districts are supposed to be based on total population, and they mostly are. But state legislative districts (unlike congressional districts) are allowed to vary in size to accommodate other concerns, like meeting the minority representation requirements of the Voting Rights Act. When the current Senate maps were new, the ideal district had 811,147 humans in it. But the biggest had 843,567 and the smallest had 778,341. Those disparities increase over the course of a decade because the growth of the state isn’t uniform.

Even districts that are drawn just right don’t stay that way for long.