Richard Wolf, USA TODAY 2:11 p.m. EST December 7, 2015

WASHINGTON — It's a simple concept that has lasted a half century: "One person, one vote." But in today's litigious society, nothing is simple.

So the Supreme Court must step in Tuesday and consider this question: When it comes to drawing election districts, who counts as a person?

On one side are civil rights groups fighting to keep the rule that emerged from a series of high court cases in the 1960s: Everyone can be counted.

On the other side are conservative groups who want tens of millions of children, non-citizens, prisoners, ex-felons and people with intellectual disabilities excluded from the count because they cannot vote.

If those challengers to Texas' state Senate districts win, many state and municipal governments from coast to coast would be sent back to the drawing boards. Urban districts with large numbers of non-voters, such as Hispanic immigrants, would grow larger, and there would be fewer of them. Many suburban and rural districts that are largely white would be smaller and more numerous. Republicans would benefit. Democrats would lose out.

“Every legislative map in the country would become presumptively unconstitutional,” says Michael Li, a lawyer at the Brennan Center for Justice at New York University School of Law, causing "upheaval ... from Vermont to California."

Congressional lines would not be affected by the court’s decision, at least until another lawsuit challenging them is filed. The Constitution requires that House seats be apportioned among states based on total population — a process by which Texas gained four seats after the 2010 Census.

Challengers to the Texas Senate districts argue that in districts with large numbers of ineligible voters, those who can vote have more power to elect the lawmakers of their choice. The opposite is true in districts with large numbers of voters — each one of them has less clout.

The case is the brainchild of Edward Blum, director of the Project on Fair Representation, who has made a practice in recent years of challenging racial and ethnic preferences in voting and higher education. He initiated the challenge to the University of Texas’ affirmative action plan that will come before the justices on Wednesday, as well as a lawsuit from Shelby County, Ala., that resulted in the court’s 2013 decision striking down the key section of the Voting Rights Act.

Blum recruited white plaintiffs Sue Evenwel and Edward Pfenninger because they live in state Senate districts with a heavy concentration of legal voters — and therefore their votes carry less weight than those in districts with large numbers of non-voters.

"In Sue Evenwel's mostly rural district, about 584,000 citizens are eligible to vote," Blum noted when bringing the case. "In a neighboring urban district, only 372,000 citizens are eligible. As a result, voters in the urban district have more sway than in the rural district; their individual electoral preferences carry more weight."

But if the justices order Texas to redraw the lines using eligible voters, some urban districts in south Texas would swell to more than 1 million people, while others in rural areas would shrink to barely 700,000.

Outside Texas, states with large numbers of undocumented immigrants would see the biggest change, such as California, New York, New Jersey, Arizona and Nevada. Cities such as Chicago and Miami also would be affected. In many other states, the rules could change but require no immediate moving of district lines because the results from using total or voter populations are not significant.

Civil rights groups contend the change from total to voter population would discriminate against minorities, who often face obstacles to voting, have more children, or lack citizenship. In addition, they say, it’s simply hard to do: While the Census Bureau provides relatively accurate population estimates every decade, there is no reliable federal data for eligible voters.

Texas wants flexibility to choose its method of drawing districts, but the Obama administration goes further and argues that the court should not give states such flexibility.

“Adopting Texas’s hypothetical approach risks rendering residents of this country who are ineligible, unwilling, or unable to vote as invisible or irrelevant to our system of representative democracy,” the solicitor general’s office argues in its brief.

Blum and his allies contend the switch would not be as dramatic as claimed – particularly if the justices require states and municipalities to take both sets of data into consideration. The court could send the case back to Texas to see if such a compromise is possible, delaying any nationwide impact.

If the court gives states the option of switching to voter population, civil rights leaders such as Sherrilyn Ifill of the NAACP Legal Defense and Educational Fund predict “mischief will happen” on the part of state legislators. Redrawing lines to exclude various groups from the head count based on age, race, ethnicity and intellectual disability could lead to more lawsuits, she says.

“I’m not exactly sure what your citizenship rights are," Ifill says, "if you don’t count and you can’t vote."

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