Until now, states have had the leeway to define their own method and have overwhelmingly opted to consider all persons; only a handful of states base apportionments on the number of voters. A ruling in favor of the challengers threatens to disqualify, at least for the purposes of representation, persons under the age of 18, documented immigrants who are not yet citizens, and former felons who have lost the right to vote. Latino immigrants and African-American males, who disproportionately live in urban areas, would be most affected.

Chief Justice Earl Warren considered the reapportionment rulings, which are now at risk of being overturned, the most important of his tenure—more important than Brown v. Board of Education, more important than Gideon v. Wainwright. They were intended to correct what had become, in effect, a system of minority rule.

Prior to the 1960s, American democracy was a deliberately misshapen enterprise. Because most states based representation in at least one chamber of their legislatures on factors other than population, district boundaries could readily be drawn to give some citizens a far greater voice than others. A lawmaker from a rural area of a state, for example, might have represented 10,000 voters, while a colleague from a more urban region might have had two times, or five times, or even ten times as many constituents. And since state lawmakers typically control districting for the U.S. House of Representatives, malapportionment guaranteed that residents of more populous districts lacked adequate representation at the federal level as well.

Most states mandated the reapportionment of at least one branch of the legislature at regular intervals, and more than half of the states required the regular reapportionment of both branches. But as urban and suburban populations swelled dramatically throughout the 20th century, lawmakers allowed district boundaries to remain fixed for decades.

Finally, the Supreme Court, led by Chief Justice Earl Warren, intervened. In a series of decisions handed down between 1962 and 1964, the Court concluded that each state’s districts for the U.S. House of Representatives must contain, as nearly as possible, the same number of people, and that both chambers in a state’s bicameral legislature must be apportioned according to the principle of “one person, one vote.” By the end of the decade, the Court had determined that apportionments for all elective municipal and county offices—including city councils, boards of supervisors, and school boards, among others—must adhere to the same standard.

In requiring that all apportionments be based on population, the Warren Court did not specify whether “persons” meant people or voters and thus the issue now before the Court. At the time—a year before the immigration act of 1965 opened the nation’s borders to millions of non-citizen residents—the distinction did not seem crucial to the justices.