The U.S. Supreme Court case United States Department of Commerce, et al. v. Montana, et al., 1991 revolved around the constitutionality of a method used by the Department of Commerce for apportioning seats in the House of Representatives among states following a census. The state of Montana challenged this method after it resulted in them losing one seat in Congress due to population shifts recorded during the 1990 Census. They argued that this violated Article I, Section 2 and Amendment XIV, Section 2 of the Constitution which mandate equal representation for equal numbers of people. However, upon review, the Supreme Court ruled against Montana with an unanimous decision (9-0). It held that while absolute mathematical precision was not achievable given geographical and political realities; minor deviations from perfect equality were acceptable as long as they furthered rational state interests or objectives such as maintaining political subdivisions' integrity or providing effective representation for sparsely populated areas.

In the dissenting opinion of the case United States Department of Commerce v. Montana, Justice O'Connor, joined by Chief Justice Rehnquist and Justices Scalia and Souter, argued that the method used by Congress to apportion seats in the House of Representatives did not violate Article I Section 2 of the Constitution. The dissenters believed that there was no constitutional requirement for absolute mathematical precision in apportionment. They pointed out that historically, Congress had always been given wide discretion to choose an appropriate method for distributing representatives among states based on population counts from each decennial census. They also noted that every previous plan adopted since 1941 had resulted in some degree of inequality between states' populations per representative due to rounding effects inherent in any system using whole numbers rather than fractions or decimals. Therefore, they concluded that this minor discrepancy did not rise to a level constituting unconstitutional discrimination against citizens living in more populous states.