New York Citys Board of Estimate consists of the mayor and two other members elected citywide, each of whom casts two votes, plus the elected presidents of the citys five boroughs, each of whom casts one vote. Appellees, residents and voters of Brooklyn, the most populous borough, charging that the city charters sections governing the boards composition are inconsistent with the Equal Protection Clause of the Fourteenth Amendment, brought suit in the District Court, which concluded that the board was a nonelective, nonlegislative body not subject to the rule established by Reynolds v. Sims, 377 U.S. 533, and other reapportionment cases. The Court of Appeals reversed, finding that the boards selection process must comply with the reapportionment cases so-called "one-person, one-vote" requirement, since its members ultimately are chosen by popular vote. On remand, the District Court determined that applying the Abate v. Mundt, 403 U.S. 182, population per representative methodology to the disparate borough populations produced a total deviation of 132.9% from voter equality among the electorates, and that the citys explanations for this range neither required nor justified such a gross deviation. The Court of Appeals affirmed, holding, inter alia, that the presence of citywide representatives did not warrant departure from the Abate methodology and, thus, that the District Courts finding of a 132.9% deviation was correct.

Held:

The Board of Estimates structure is inconsistent with the Equal Protection Clause of the Fourteenth Amendment because, although the boroughs have widely disparate populations, each has equal representation on the board. Pp. 692-703.

(a) Board membership elections are local elections subject to review under the prevailing reapportionment doctrine. The board, composed of officials who become members as a matter of law upon their elections, has a significant range of fiscal and legislative functions common to municipal governments, including assisting in the formulation of the [489 U.S. 688, 689] citys budget, and controlling land-use, contract, and franchise powers. That the citywide members enjoy a 6-to-5 voting majority does not render the boards composition constitutional, since the borough presidents control the outcome of board decisions anytime the citywide members do not vote together and always control budgetary decisions because the mayor has no vote on such matters. Moreover, the Reynolds-Abate approach should not be put aside in favor of the theoretical Banzhaf Index - which produces a standard deviation of 30.8% for nonbudget matters and a larger figure for budget items by mathematically calculating a voters power to determine the outcome of an election - since the latter approach tends to ignore partisanship, race, voting habits, and other characteristics having an impact on general election outcomes. Pp. 692-699.

(b) The presence of citywide members is a major component to be factored into the process of determining the deviation between more or less populous boroughs. This approach - which yields a standard deviation of 78% - recognizes that voters in each borough vote for, and are represented by, both their borough president and the citywide members, thus departing from the lower courts approach which treated the five boroughs as single-member districts, each with a representative having a single vote. Pp. 699-701.

(c) The citys proffered governmental interests - that the board is essential to the successful government of New York City, is effective, and accommodates natural and political boundaries as well as local interests - do not suffice to justify a 78% deviation from the one-person, one-vote ideal, particularly because the city could be served by alternative ways of constituting the board that would minimize the discrimination in vo