Meaning of 'one person, one vote' at stake at Supreme Court

The legal challenge is being financed by Edward Blum, whose Project on Fair Representation also is behind a Texas affirmative action challenge that will be argued before the high court on Wednesday, as well as the lawsuit that led to the 2013 decision that wiped away a key element of the federal Voting Rights Act.

In a dispute from Arizona, the justices are weighing whether even small differences in population among districts are appropriate if they are done for partisan advantage or to comply with the now-nullified advance-approval requirement in the Voting Rights Act.

Arizona voters who are challenging the decisions of an independent redistricting commission claim that Democrats benefited from the legislative district boundaries.

The case brought by Texas residents Sue Evenwel and Edward Pfenninger highlights the difference in eligible voters in the mainly rural districts outside Houston where they live, and those in a downtown Houston district with equal population, but at least 170,000 fewer people eligible to vote.

The only reference to population and political districts in the Constitution requires the use of the once-a-decade census as the basis for divvying up congressional districts among the states, said Stanford University law professor and political scientist Nathaniel Persily.

The annual American Community Survey reaches 3.5 million U.S. households each year and helps political line-drawers ensure that districts comply with the Voting Rights Act, said Peter Morrison, former director of the RAND Corp.'s Population Research Center, and other demographers.