Mr. Justice White
delivered the opinion of the
This appeal is'the latest step in the long and fitful attempt. to devise a constitutionally valid reapportionment scheme for the State of Arizona. For the reasons given, we affirm the judgment of the District Court.
In April 1964, shortly-before this Court’s decision in Reynolds v. Sims, 377 U. S. 533 (1964), and in its companion cases, suit was filed in the District Court for the District of Arizona attacking the then-existing state dis-tricting laws as unconstitutional. Following those decisions, the three-judge District Court ordered all proceed-; ings stayed “until the expiration of a period of 30 days-next following adjournment of the next session” of the Arizona Legislature. (App. 2-3, unreported.) Nearly ' a year later, on May. 18, 1965, after the legislature had failed to act, the court again deferred trial pending a special legislative session called by the Governor to deal with the necessity of reapportionment. The special session enacted Senate Bill 11, which among other things provided one senator for a county of 7,700 and another for a county of 55,000. The session did not undertake to reapportion the House. Trial was had in November 1965 and on February 2, 1966, the court enjoined enforcement of Senate Bill, 11, which, it held, “bears'evidence of having been thrown together .as a result of considerations wholly apart from those laid down as compulsory-by the decisions of the Supreme Court.” Klahr v. Goddard, 250 F. Supp. 537, 541 (Ariz. 1966). The plan, said the court, was “shot through with invidious discrimination.” Id., at 546. The court also held that the existing House plan produced disparities of nearly four to one, which was clearly impermissible under our decisions.
Noting that the legislature “has had ample opportunity” to produce a valid reapportionment plan, the court formulated its own plan as a “temporary and provisional reapportionment,” designed to govern the impending preparation for the 1966 elections. The plan was to be in effect “for the 1966 primary and general elections and for such further elections as may follow until such time as the Legislature itself may adopt different and valid plans for districting and reapportionmerit.” Id., at 543. It retained jurisdiction, as it has done since.
Some 16 months later, in June 1967, the Arizona Legislature enacted “Chapter 1, 28th Legislature,” which again attempted reapportionment of the State. Within the month, suit was filed charging that this Act also was unconstitutional, but the court deferred action pending the outcome of a referendum scheduled with the November 1968 election for the legislature and Congress. It ordered those elections to be held in accordance with its own 1966 plan, as supplemented. Klahr v. Williams, 289 F. Supp. 829 (Ariz. 1967). The legislative plan was approved by the voters in the referendum and signed into law by the Governor on January 17, 1969. A hearing on the plan was commenced the same day. The court concluded on July 22, 1969, that the plan, which set up “election districts” based on population and “legislative” subdistricts based on voter registration, would allow deviations among the legislative subdistricts of up to 40% from ideal until 1971, and up to 16% thereafter. The court properly concluded that this plan was invalid under Kirkpatrick v. Preisler, 394 U. S. 526 (1969), and Wells v. Rockefeller, 394 U. S. 542 (1969), since the legislature had operated on the notion that a 16% deviation was de minimis and consequently, made no effort, to achieve greater equality. The court ordered its 1966 plan continued once again “until the Legislature shall have adopted different, valid, and effective plans for redistricting and reapportionment . . . .” (App. 85, unreported.) It refused to order the 1970 elections to be held at large, since there was “ample time” for the legislature “to meet its obligation” before the machinery for conducting the 1970 elections would be engaged.
' The legislature attempted a third time to enact a valid plan. It passed “Chapter 1, House Bill No.- 1, 29th Legislature,” which was signed. into law by the Governor on. January 22, 1970, and which is the plan involved in the decision from which this appeal is taken. Appellant challenged the bill, alleging that it “substantially disenfranchises, unreasonably and unnecessarily, a large number of the citizens of the state,” App.-106, and “creates legislative districts that are grossly unequal.” App. 108. Appellant at that time submitted his own plan for the court’s consideration. Appellant’s primary dispute with the new plan was that it substantially misconceived the current population distribution in Arizona. The court agreed that appellant’s plan, which utilized 1968 projections of 1960 and 1965 Arizona censuses, could “very likely [result in] a valid reapportionment plan” but it declined to implement the plan, since it was based on census tracts, rather than the existing precinct boundaries, and “the necessary reconstruction of the election precincts could not be accomplished in time” to serve the ■ 1970 ■ election, whose preliminary preparations were to begin in a few weeks. Klahr v. Williams, 313 F. Supp. 148, 150 (Ariz. 1970). At the same time, the court observed that its 1966 plan had fallen béhind contemporary constitutional requirements, due to more recent voter registration data (which increased the deviation between high and low districts to 47.09%) and the intervening decisions of this. Court in Kirkpatrick and Wells, supra, and Burns v. Richardson, 384 U. S. 73 (1966).
Turning to the legislature’s plan, the court found it wanting in several respects. First, though the result indicated population deviation between high and low districts of only 1.8%, the population formula used did not “truly represent the population within [the] precincts in either 1960 or 1968,” and thus “the figures produced . . . are not truly population figures.” 313 F. Supp., at 152. Second, the computer that devised the plan had been programmed to assure that the plan would not-require any -incumbent legislator to face any other incumbent for re-election. Third, the programming gave priority to one-party districts over districts drawn without regard to party strength. The court held that “the incumbency factor has no place in any reapportionment or redistricting” and found “inapposite” the “consideration of party strength as a factor . . . .Ibid.
The court was thus faced with a situation where both its 1966 plan and the legislature’s latest attempt fell short of the constitutional standard. At. that time, however, the 1970 elections were “close at hand.” The court concluded that another legislative effort was “out of the question” due to the time and felt that it could not itself devise a new plan without delaying primary elections, “a course which would involve serious risk of confusion and chaos.” Ibid. It considered at-large elections, but the prospect of electing 90 legislators at large was deemed so repugnant as to be justified only if the legislature’s actions had been “deliberate and inexcusable”; the court instead believed that the large population increase in Arizona since the last reliable census in 1960 was more to blame. Concluding that the 1970 elections would be the last to be held .before the 1970 census data became available for new plans, the court chose what it considered the lesser of two evils and ordered the elections to be conducted under the legislature’s plan. In its order to this effect, the court noted that it “assumes that the Arizona Legislature will by November 1, 1971, enact a valid plan of reapportionment,” but that “[u]pon failure of the Legislature so to do, any party to this action may apply to the court for appropriate relief.” Id., at 154.
The state officials did not seek review of the District Court’s judgment declaring Chapter 1 unconstitutional. Appellant, however, appealed to this Court. His notice of appeal was filed on June 18,'1970, his jurisdictional statement on August 17, 1970. The latter presented the single question whether it was error for the United States District Court to refuse to enjoin the enforcement of the Arizona Legislature’s most recent effort to reapportion the State. Appellees’ motion to dismiss or affirm was filed on November' 24. We noted probable jurisdiction on December 21, 400 U. S. 963.
Meanwhile, the 1970 elections were* held in accordance with the District Court’s decree. Appellees suggest that the issue presented is moot and appellant concedes “the 1970 general election has already been held so that that aspect of the wrong cannot be remedied.” Brief 8. But appellant now argues that however that may be, the District Court should now proceed to adopt a plan of reapportionment which would be displaced only upon the adoption of a valid plan by the legislature. Appellant doubts that postponing judicial action until after November 1 will give the District Court sufficient time, prior to June 1972, when the election process must begin in Arizona, to consider the. legislative plan and to prepare, its own plan if the legislative effort does, not comply with the Constitution. The feared result is that another election under an unconstitutional plan would be held in Arizona.
Reapportionment history in the State lends some substance to these fears, but as we have often noted, district-ing and apportionment are legislative tasks in the first instance, and the court did not err in giving the legislature a reasonable time to act based on the 1970 census figures which the court thought would be available in the summer of 1971. We agree with appellant that the District Court should make very sure that the 1972 elections are held under a constitutionally adequate apportionment plan. But the District Court knows better than we whether the November 1 deadline will afford it ample opportunity to assess the legality of a new apportionment statute if one is forthcoming and to prepare its own plan by June 1, 1972, if the official version proves insufficient. The 1970 census figures, if not now available, will be forthcoming soon; and appellant, if he is so inclined, can begin to assemble the necessary information and witnesses and himself prepare and have ready for submission what he deems to. be an adequate apportionment plan. Surely, had a satisfactory substitute for Chapter 1, held unconstitutional by the District Court, been prepared and ready the court would have ordered the 1970 elections held under that plan rather than the invalid legislative scheme. . And surely if appellant has ready for court use on November 1, 1971, a suitable alternative for an unacceptable legislative effort, or at least makes sure that the essential information is on hand, there is no justifiable ground for thinking the District Court could not, prior to June 1, 1972, complete its hearings and consideration of a new apportionment statute and, if that is rejected, adopt a plan of its own for use in the 1972 elections. Nor do we read the District Court decree as forbidding appellant from petitioning for reopening of the case prior to November 1, 1971, and presenting to the District Court the problem which it has now raised here but which we prefer at this juncture to leave in the hands of the District Court. The judgment is affirmed.
It is so ordered.
Throughout this litigation, congressional districting has been at issue as well and has suffered the same fate as reapportionment of the legislature. However, appeal has been • taken , here only with respect to the lower court’s decree goncerhing legislative reapportionment.
The court issued two supplemental decrees in 1966 which modified and clarified .the original order. 254 F. Supp. 997, 289 F. Supp. 827.
Apparently under Arizona law, a referendum is required before a bill can become law where, as here, sufficient signatures against the bill are filed with the Secretary of State. See Klahr v. Williams, 289 F. Supp. 829 (Ariz. 1967).
“The population factor in each of the election precincts comprising part of a legislative'district was obtained by instructing the computer to take the 1968 voter registration for the precinct and divide it by the 1968 voter registration for the county in which the precinct was located, thereby obtaining the percentage of registered voters of the county residing within the precinct. The computer was then directed to multiply that percentage figure by the 1960 census for the county in which the precinct was located, thereby obtaining the population factor for the precinct.” 313 F. Supp., at 151-152.
Though wé noted in Burns v. Richardson, 384 U. S. 73, 89 n. 16, that “[t]he fact that district boundaries may have been drawn in' a way that- minimizes the number of contests between present incumbents does not in and of itself establish invidiousness,”' it is sufficient to note here, that the District Court did not base' its decision solely on this factor.
E. g., Reynolds v. Sims, 377 U. S. 533, 586 (1964):
“[Legislative reapportionment is primarily a matter for legislative consideration and determination, and . . judicial relief becomes appropriate only when a legislature fails to reapportion according to federal constitutional requisites in a timely fashion after having had an adequate opportunity to do so.”
Appellant has contended here that the use of voter registration figures, rather than actual population, to determine district size operates to the detriment of the poor, blacks, Mexican-Americans, and American Indians. In light of our disposition of this case, we need only advert to our admonition in Burns v. Richardson, supra, that use of voter registration as a basis may “perpetuate under-representation of groups constitutionally entitled to participate in the electoral process,” 384 U. S., at 92, and is allowable only if it produces “a distribution of legislators not substantially different from that which would have resulted from the use of a permissible population basis.” Id., at 93. We presúme, of course, that any plan submitted, and certainly any- plan approved by the District Court, will be faithful to this requirement.