Article I, 2, of the Constitution requires apportionment of Representatives among the States "according to their respective Numbers." A 1941 federal statute provides that after each decennial census "the method known as the method of equal proportions" shall be used to determine the number of Representatives to which each State is entitled. Application of that method to the 1990 census caused Montana to lose one of its two seats in the House of Representatives. If it had retained both seats, each district would have been closer to the ideal size of a congressional district than the reapportioned single district. The State and several of its officials (hereinafter Montana) sued appropriate federal defendants (hereinafter the Government) in the District Court, alleging, inter alia, that the existing apportionment method violates Article I, 2. A three-judge court, convened pursuant to 28 U.S.C. 2284, granted Montana summary judgment on this claim, holding the statute unconstitutional because the variance between the single districts population and that of the ideal district could not be justified under the "one-person, one-vote" standard developed in Wesberry v. Sanders, 376 U.S. 1, and other intrastate districting cases.

Held:

Congress exercised its apportionment authority within the limits dictated by the Constitution. Pp. 447-466.

(a) The general admonition in Article I, 2, that apportionment be made "according to [the States] respective numbers" is constrained by three constitutional requirements: the number of Representatives shall not exceed one for every 30,000 persons; each State shall have at least one Representative; and district boundaries may not cross state lines. In light of those constraints and the problem of fractional remainders - i.e., the fractional portion of the number that results when the States total population is divided by the population of the ideal district must either be disregarded or treated as equal to one Representative because each State must be represented by a whole number of legislators - Congress has considered and either rejected or adopted various apportionment methods over the years, the most recent method tried being the method of equal proportions, also known as the "Hill Method." A National Academy of Sciences committee recommended that method as the fairest of the five [503 U.S. 442, 443] methods the committee felt could lead to a workable solution to the fractional remainder problem. If Congress had chosen the method of the harmonic mean, also known as the "Dean Method," Montana would have received a second seat after the 1990 census. Pp. 447-456.

(b) This Court rejects the Governments argument that Congress selection of any of the alternative apportionment methods presents a "political question" that is not subject to judicial review under the standards set forth in Baker v. Carr, 369 U.S. 186, 217. Significantly, the Government does not suggest that all congressional decisions relating to apportionment are beyond judicial review, but merely argues that the District Court erred in concluding that the Constitution requires the greatest possible equality in the size of congressional districts, as measured by absolute deviation from ideal district size. Thus, the controversy here turns on the proper interpretation of the relevant constitutional provisions. As in Baker itself and the apportionment cases that followed, the political question doctrine does not place this kind of constitutional interpretation outside the proper domain of the Judiciary. Pp. 456-459.

(c) Congress had ample power to enact the statutory procedure at issue and to apply the Hill Method after the 1990 census. It is by no means clear that the facts here establish a violation of the Wesberry one-person, one-vote standard. Although Montanas evidence demonstrated that applic