The Supreme Court, in its ruling Wednesday, said any flaws in the Arizona redistricting plan probably resulted from a well-intentioned attempt to comply with the Voting Rights Act. (Karen Bleier/AFP/Getty Images)

The Supreme Court ruled unanimously Wednesday 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 [at first look] case of invidious discrimination.”

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.

Wednesday’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 brought the challenge. They said that the Arizona Independent Redistricting Commission had underpopulated some districts and overpopulated 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.

“We believe that attacks on deviations under 10% will succeed only rarely, in unusual cases,” Breyer wrote.

The ruling upheld a decision by a three-judge panel in Arizona that the population deviations “were primarily a result of good-faith efforts to comply with the Voting Rights Act [VRA] . . . even though partisanship played some role.”

Breyer said the commission — composed of two Republicans, two Democrats and one independent — was trying to fulfill a provision of the VRA that does not allow diminishing the number of districts in which minority groups can elect their “preferred candidate of choice.”

Over the objections of the Republican members, the other commissioners moved some white voters out of two districts to increase the percentage of Hispanic voters in each.

It probably is true that Democratic-leaning districts are underpopulated and Republican ones are overpopulated, Breyer wrote. “But that fact may well reflect the tendency of minority populations in Arizona in 2010 to vote disproportionately for Democrats. If so, the variations are explained by the commission’s efforts to maintain at least 10” districts where minorities could elect candidates of their choice, he added.

And Breyer pointed out that at the time, Arizona was among the states — the rest of them mostly in the South — that were under special scrutiny and required to have their redistricting plans approved in advance by Justice Department officials or judges in Washington.

In a divisive decision in 2013, the Supreme Court excised the list of states covered by the requirement. Congress has not attempted to come up with a new one, a matter of contention between Democrats and Republicans.

Wednesday’s decision came in Harris v. Arizona Independent Redistricting Commission.