The Supreme Court rejected a conservative challenge to the 'one person, one vote' rule that counts non-voters such as children and prisoners. Here's what you need to know. (Victoria Walker/The Washington Post)

Update: The Supreme Court has done Democrats a solid, ruling in the Evenwell case that states may use total population, which includes non-voters such as children and others not eligible to vote, when districts are being created. All states use total population now, with some minor exceptions. The Supreme Court was considering a challenge from Texas, where a conservative voting rights group said only the citizen voting age population should be used, which would change the state's Senate districts dramatically. 

Last year, we looked at the "one person, one vote" principle works and how changing this standard -- to include only eligible voters -- could have helped Republicans. Below is our primer on what was at stake in the case and how an alternate ruling would have changed things. The following post is from May:

The Supreme Court is planning to hear a case that could turn how we vote for our elected officials on its head -- and possibly help Republicans in a significant way.

The case, involving the principle of "one person, one vote," has to do with how our electoral districts are formed. Justices announced they will consider whether most states' current practice of counting the entire population to create those districts -- rather than just eligible voters -- is constitutional. That could shift many districts' political power to more suburban and rural districts, which tend to have older, whiter and more Republican voters.

The news made waves among election law scholars, but it could have a much broader impact on politics in general. Here's what you need to know.

What does the law say now?

In 1964, the Supreme Court said states have to size districts based on population, not geography. A district in rural Nevada is several times larger than the district that represents Las Vegas but represents almost exactly the same number of people, for example.

NVcongdistricts
Courtesy National Atlas and Wikipedia

But the Supreme Court left out one important deciding factor: Judges didn't specify whether states should count total population or just eligible voters. Indeed, a 1966 case involving Hawaii left it up to states when it comes to whether to include undocumented immigrants, felons and others ineligible to vote. That's what the court will look at 50 years later: Whether people who can't vote, which also includes those under 18 years old, should be considered when drawing electoral districts.

The federal government uses general population Census data to give states the number of congressional districts. Today, most states use total population regardless of voter eligibility to draw the lines for those districts. That includes Texas, which was where the case the Supreme Court decided on Monday centered. This is what has been dubbed "one person, one vote."

Wait, I wasn't listening. What is 'one person, one vote?'

"One person, one vote" is the requirement that each district has a roughly equal number of people in it, but it's not clear whether it refers to total population or some other metric, like eligible voters.

Got it. So what's the challenge to the law?

Just one group is challenging  the population-counting method for creating districts. The conservative Project on Fair Representation sued on behalf of an official in the Texas Republican Party, who noted that the state's equal-population state Senate districts vary widely when counting  eligible voters.

The Project on Fair Representation has challenged Affirmative Action cases in the past. Its leader, Edward Blum, told The Post's Robert Barnes that drawing districts based on eligible voters is just more equitable.

“This case presents the court with the opportunity to restore the important principle of one-person, one-vote to the citizens of Texas and elsewhere,” Blum said in a statement to Barnes.

The justices at least agreed it's worth a look -- a surprising move to many legal scholars. The court said it will consider whether states that count non-voters to create election districts "deny voters an equal right." The case will likely be argued this fall.

But if most states agree and one group doesn't, why did the court take this up?  

As Barnes noted, it's surprising this case is being taken up because there's almost no disagreement about the law in lower courts.

But law professors like NYU's Richard Pildes say it makes sense. Having equal representation in political bodies is such a basic constitutional right that it's frankly surprising the court has left it up to the states to decide how to do it, Pildes wrote in a listerv with other law professors.

"And as long as the baseline remains constitutionally undefined, states can manipulate the districting system by choosing one baseline over another in order to achieve various partisan or political ends." Pildes said.

What's more, states do something totally different when making sure they comply with the federal Voting Rights Act. They use use eligible voters -- not total population -- to ensure electoral districts are set up so minorities have an equal chance to run for office. By contrast, the Census-based, once-a-decade allocation of the number of congressional districts to each state (i.e. Nevada gets four districts, California gets 53, etc.) is based on total population.

The Supreme Court might want to settle the tension between the two laws.

So what could happen if states count only eligible voters?

A court ruling changing "one person, one vote" to eligible voters (or some other metric) could flip many district sizes on their head. It would end up giving rural and more homogeneous districts more power than diverse urban areas, where there tend to be fewer eligible voters. As we'll discuss later, this likely accrues to the benefit of Republicans.

In California, for example, Claremont McKenna College government fellow Doug Johnson noted on his Facebook page that high-diversity districts such as the 40th in Los Angeles County have substantially fewer eligible voters than the whiter, rural section of the state represented by the northern 1st District. Measured by population, the two districts are equal in size. Measured by eligible voters, the northern 1st District is twice as big as the L.A. area's 40th (roughly 521,000 to 261,000).

This is among the most extreme examples, but it's clear that districts in many parts of the country would have to be redrawn to a significant extent. Here's something we wrote last year noting the huge differences in eligible voters per district across the country -- from 778,000 in Montana's at-large district to the fewer than 300,000 in California's 40th.

The older and white districts like California's 1st and Montana's at-large district are held by Republicans; California's 26th is a Democratic stronghold.

What about states and districts with high numbers of undocumented immigrants? This would affect them a lot, right?

Yep. Counting only eligible voters would take away political power from states with high Hispanic populations such as Texas, California, Florida and Nevada. But it's not just because minorities tend to be less politically active; it's also because the Latino population skews very young -- and those under 18 aren't eligible to vote.

Flipping those districts would have an immediate impact on Democrats, who would see districts with lots of Latinos but fewer eligible voters be redrawn to take in more voters -- thus reducing the number of heavily Latino districts or making them more difficult to hold by pulling in more white (i.e. more Republican) voters. In November's 2014 midterm elections, the New York Times found that Latinos voted for Democrats by a margin of two-to-one. And they are actually one of the few groups that voted more heavily for President Obama in 2012 than in 2008.

All that's to say both parties could see their strategies for engaging Latino voters, for drawing electoral districts and for registering voters upended if the Supreme Court decides to change the way we draw our districts.

Update: As we now know, the court did not decide to do that, which means our electoral districts will stay the same for the foreseeable future, and so will the balance of political power in them.