The geographical boundaries of legislative districts across the country are redrawn to reflect population changes measured by the census conducted by the federal government every decade. In most states, redistricting is done by the party in power.

Though both parties have engaged in partisan gerrymandering, President Donald Trump's fellow Republicans have been the primary beneficiaries since the last round of redistricting following the 2010 census.

Election reformers had hoped the justices would intervene to stop political parties in power at the state level from using electoral maps to further cement their majorities and dilute the voting clout of people who support rival parties.

They warned that gerrymandering is becoming more extreme and can better engineer election outcomes with the use of precise voter data and powerful computer software, and will get worse if courts cannot curb it.

In the decision, Roberts said the court was not condoning excessive gerrymandering, which can yield election results that "seem unjust," but added that it is an inherently political act reserved for legislatures, not courts, whose review would appear political.

The Constitution does not allow courts to forbid the practice, like it does racial discrimination in political map-drawing, Roberts said. "You can take race out of politics," Roberts said from the bench, "but you can't take politics out of politics."