can incumbents to run against each other and to draw in as many Republican voters as possible in the process." Ibid.
The drafters' efforts at selective incumbent protection "led to a significant overall partisan advantage for Democrats in the electoral maps," with "Republican leaning districts . . . vastly more overpopulated as a whole than Democratic leaning districts," and with many of the large positive population deviations in districts that paired Republican incumbents against each other. Id., at 1331. The District Court found that the population deviations did not result from any attempt to create districts that were compact or contiguous, or to keep counties whole, or to preserve the cores of prior districts. Id., at 1331–1334. Rather, the court concluded, "the population deviations were designed to allow Democrats to maintain or increase their representation in the House and Senate through the underpopulation of districts in Democratic leaning rural and inner city areas of the state and through the protection of Democratic incumbents and the impairment of the Republican incumbents' reelection prospects." Id., at 1334. The District Court correctly held that the drafters' desire to give an electoral advantage to certain regions of the State and to certain incumbents (but not incumbents as such) did not justify the conceded deviations from the principle of one person, one vote. See Reynolds v. Sims, 377 U.S. 533, 565–566 (1964) (regionalism is an impermissible basis for population deviations); Gaffney v. Cummings, 412 U.S. 735, 754 (1973) ("[M]ultimember districts may be vulnerabl[e] if racial or political groups have been fenced out of the political process and their voting strength invidiously minimized"). See also Reynolds, 377 U.S., at 579 (explaining that the "overriding objective" of districting "must be substantial equality of population among the various districts" and that deviations from the equal population principle are permissible only if "incident to the effectuation of a rational state policy").
In challenging the District Court's judgment, appellant invites us to weaken the one person, one vote standard by creating a safe harbor for population deviations of less than 10 percent, within which districting decisions could be made for any reason whatsoever. The Court properly rejects that invitation. After our recent decision in Vieth v. Jubelirer, 541 U.S. 267 (2004), the equal population principle remains the only clear limitation on improper districting practices, and we must be careful not to dilute its
