Fifty-one years ago the Supreme Court handed down its one-person-one-vote decision, requiring that within each state congressional and legislative districts must have equal populations.
That gave redistricters a relatively easy standard to meet. Census data provides block-by-block population counts every 10 years, and it's possible now to draw lines for districts so that their populations are identical or vary by just one person.
But redistricting cases keep making their way to the Court nonetheless.
One reason is that the Voting Rights Act amendments of the 1980s have been interpreted as requiring the creation of a maximum number of districts with majorities or near-majorities of black or Hispanic residents. This has produced many grotesquely shaped constituencies and much litigation.
This month the Court heard two other redistricting cases. One, Evenwell v. Abbott, was brought in Texas. The plaintiffs argue that districts should be equal not in total population but in number of eligible voters. They live in areas where almost all adult residents are citizens, but in other areas -- the Lower Rio Grande Valley particularly -- a majority of residents aren't, because they are non-citizens or children.
As a result some districts have two to three times as many eligible voters as others. That's not equal representation, plaintiffs argue.
This was not a situation foreseen in 1964. Due to restrictive immigration laws, depression and war, the nation's population had the lowest percentage of non-citizens since the 1830s. And in that baby boom era, just about every part of the country had similar percentages of adults and children. Equal population districts thus tended to generate districts with equal numbers of eligible voters.
Plus, the courts have made exceptions to the equal-population rule, so that prison populations and military bases are not counted for state legislative representation. You don't want to create a district where no one is eligible to vote.
Today some heavily immigrant districts come close to that. California's congressional districts had equal total populations in the 2010 Census. But in November 2012, 337,634 people voted in the low-immigrant-population 4th district in the northwest Sierra and only 119,234 in the heavily Hispanic 21st district in the Central Valley.
So there's an intellectually respectable argument that districts should be based on eligible voter population rather than total population. Or, as Justice Anthony Kennedy suggested, could be. Texas' position is that its Republican legislature chose to use total population (even though using eligible voters would help Republicans) and that that is at least permissible as well, if not mandatory.
There's one other problem for the Evenwell plaintiffs. The Census taken every 10 years provides fine grain detail on total population. The Census' American Community Survey and other sources provide estimates and less detail. That leaves more room for districters to draw lines for partisan advantage -- gerrymandering.
So it seems unlikely that the Court will require use of eligible voters. But if a majority of justices indicate they'll permit it, that could have repercussions -- mostly in favor of Republicans -- in redistricting following the 2020 Census or if, as permitted, they redraw their lines before that.
The other redistricting case heard last month, Harris v. Arizona Independent Redistricting Commission, raises the issue of whether a plan in which district populations vary by 10 percent can be invalidated because it favors one party. Here the Court has an opportunity to set a standard that will cut back on its redistricting caseload.
Arizona's reputedly nonpartisan commission, like California's, was successfully stacked by Democrats and drew lines that tended to pack large numbers of Republicans into a few districts with above-average populations. The commission says it acted to align districts with county lines, as if legislators are supposed to represent governmental units rather than people.
In 2012, Republicans carried Arizona's vote for U.S. House of Representatives by a 52 to 44 percent margin. But under the commission's plan Republicans won only four districts and Democrats five. Winning Republicans won 61 to 67 percent of the vote. Three winning Democrats won margins averaging of 2.9 percent.
The Court has refused to overturn even blatantly partisan plans, notably one by Pennsylvania Republicans. But allowing districts to be 10 percent larger or smaller than average provides redistricters with far greater opportunity for gerrymandering than a requirement that districts come very close to equal populations.
Courts can't stop partisan gerrymandering entirely. But they can limit its effectiveness by enforcing strict numerical equality, by simple arithmetic. Supreme Court justices weary of redistricting cases might keep that in mind.