The Washington Post

WASHINGTON — The Supreme Court ruled unanimously yesterday that an independent redistricting commission in Arizona did not improperly favor Democrats when drawing the state’s legislative districts.

The justices said variances in the populations of the districts, drawn after the 2010 Census, were not so great as to violate the Constitution’s rule of “one person, one vote.”

The court added that any flaws in the redistricting plan are more likely to have resulted from a well-intentioned attempt to comply with the Voting Rights Act, rather than to favor one party over another.

“We have made clear that the Constitution, while insisting upon compliance with the principle of one-person, one-vote, does not demand mathematical perfection,” Justice Stephen G. Breyer said in announcing the decision. “We have said that minor deviations from mathematical equality do not by themselves make out a prima facie case of invidious discrimination.”

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It was the court’s second ruling in recent weeks on the subject of one-person, one-vote. It previously decided in a case from Texas that states may comply with the Constitution by counting all residents of a place, rather than just the voting-age population, when drawing electoral districts.

And last term, the court upheld the decision of Arizona voters to put redistricting decisions in the hands of the independent commission, rather than the legislature.

Yesterday’s decision was a slim one, and its unanimity suggested it was more a restatement of the court’s precedents than a ruling breaking new ground.

Arizona Republicans who brought the challenge said that the Arizona Independent Redistricting Commission had under-populated some districts and over-populated others in a way that packed white Republicans into fewer districts and minimized their impact.

The difference between the least-populous district and the largest was a little more than 8 percent. But Breyer explained that the Supreme Court has previously held that legislative districts may vary by as much as 10 percent before they are thought to violate the one-person, one-vote principle.