Article 1 Sec. 2

The One Man, One Vote, or equal population requirement for congressional districts derives from this clause in the U.S. constitution that Representatives be chosen "by the People of the several States" means that, as nearly as is practicable, one person's vote in a congressional election is to be worth as much as another's. (Wesberry v. Sanders, 376 U.S. 1 (1964). The Supreme Court derived this mandate from speeches, debates and discussion at the constitutional convention.

14^th Amendment – Equal Protection Clause

One Man, One Vote also applies to state legislative maps and any local government boundaries in which public officials are selected by district. The clause, which reads "nor shall any State [...] deny to any person within its jurisdiction the equal protection of the laws," is interpreted by the Supreme Court to require equally populated state legislative and local districts, albeit with considerably more wiggle room for deviations.

A map that has large enough population variance between districts could trigger lawsuits to compel a state to rectify the deviations. These suits are also known as “malapportionment” suits referring to the population imbalances. Generally, these lawsuits occur when a state has not redrawn district boundaries in the expected amount of time after the census is completed.  Malapportionment suits can also be lodged against newly drawn maps if there an objectionable level of population deviations.

Generally speaking, if a congressional map deviates by more than few persons, litigation can be instituted in a court, which will require the state defendants to offer a rational state policy as to why the deviations exist. In the case of state or local district maps, most malapportionment lawsuits challenge maps with an overall deviation above 10%. Defendants in these cases have the burden of showing that a rational policy justifies the excessive deviations.

This quote from a recent Supreme Court case sums up equal population litigation quite well:

"States must draw congressional districts with populations as close to perfect equality as possible. See Kirkpatrick v. Preisler, 394 U. S. 526, 530–531 (1969). But, when drawing state and local legislative districts, jurisdictions are permitted to deviate somewhat from perfect population equality to accommodate traditional districting objectives, among them, preserving the integrity of political subdivisions, maintaining communities of interest, and creating geographic compactness. See Brown v. Thomson, 462 U. S. 835, 842–843 (1983). Where the maximum population deviation between the largest and smallest district is less than 10%, the Court has held, a state or local legislative map presumptively complies with the one-person, one-vote rule. Ibid.2 Maximum deviations above 10% are presumptively impermissible." (Evenwel v. Abbott, 578 136 S.Ct. 1120, 1124 (2016)

Equal Protection and Gerrymandering: In addition to the one-man, one-vote requirement, the Equal Protection Clause is the source of racial gerrymandering claims and at one time, partisan gerrymandering lawsuits. Racial gerrymandering legal doctrine was developed in 1990’s beginning with the famous Shaw line of cases based on Shaw v. Reno (509 U.S. 630 (1993)). These cases often featured highly irregular district shapes, which the court took as proof (along with other evidence) that the disputed districts were drawn predominantly on the basis of race.

Once a court makes a finding that a district or districts were formed mostly using race, a defendant must do more than offer a rational state policy – as is the case in equal population cases. Instead, the court invokes “strict scrutiny” and a defendant must show that the map or district is “narrowly tailored to further a compelling governmental interest.”

As you might imagine, there are very few “compelling state interests” to justify race-based line-drawing. Compliance with the Voting Rights is one. In many of these Shaw cases, however, the court either found that the line-drawing was not “narrowly tailored” to achieve VRA compliance, or there was no compliance issue to begin with. The reasoning for the court’s tough treatment of race-based line-drawing is best summed up in in this quote from Shaw:

Classifications of citizens based solely on race are by their nature odious to a free people whose institutions are founded upon the doctrine of equality because they threaten to stigmatize persons by reason of their membership in a racial group and to incite racial hostility. Thus, state legislation that expressly distinguishes among citizens on account of race-whether it contains an explicit distinction or is "unexplainable on grounds other than race,. . .must be narrowly tailored to further a compelling governmental interest. . . . Redistricting legislation that is alleged to be so bizarre on its face that it is unexplainable on grounds other than race demands the same close scrutiny, regardless of the motivations underlying its adoption. (Shaw at 631)

Partisan gerrymandering had also been a claim active in courts since the 1980’s until recently in 2019. The claim was that voters in gerrymandered districts were discriminated against based on their party affiliation if the gerrymandering kept their political party from having a reasonable chance at success in a district or in the legislature as a whole. For decades there was some consternation around how any court would be able to define and consequently measure, the level of partisan line-drawing that would violate the Equal Protection Clause. The Supreme Court during this period was deeply divided on whether or not a workable definition and measurement existed. If it did not, they all agreed – the legal claim would be a nonjusticiable political question, that is, not something any court could settle.

Every since a plurality of the court agreed it might be “possible” to adjudicate, many scholars and academics worked hard to help establish a measure for partisan gerrymandering – a measure that could be used to distinguish acceptable gerrymandering from unconstitutional gerrymandering. This distinction was important because courts have long recognized that redistricting has always been the purview of the political party in power, and since there is no constitutional requirement for proportionate representation, the majority party will likely seek advantage in redistricting.

In the end, the court gave its search for a partisan gerrymandering standard in 2019 when a pair of partisan gerrymandering cases were declared nonjusticiable political questions. (Rucho v. Common Cause, 588 US _ (2019)) What this means for litigation, is that the partisan gerrymandering claim is a dead letter in federal courts.