Richard Winger helpfully points out some confusion about the effects of Evenwel on reapportionment. So let’s make this clear.
1. At issue in Evenwel, is whether drawing district for state legislatures and other state purposes must be done on total (eligible or registered) voter basis, as opposed to a choice between that, total population, or perhaps other measures, to comply with the U.S. Constitution’s 14th Amendment equal protection clause.
2. The source of the one person, one vote rule in congressional elections within is Article 1, not the Equal Protection Clause. (See Wesberry v. Sanders.) The rules are somewhat different. OPOV for congressional districts must be exactly equal, but there is some room for minor deviations in state (and local) redistricting. Nonetheless, if the Court in Evenwel requires use of voters in the denominator for OPOV purposes in state elections, its logic would likely carry over to the OPOV rule for congressional elections.
3. Nonetheless, the number of congressional districts across states (commonly called apportionment) is textually committed in the Fourteenth Amendment to one based on total numbers of people. Section 2 reads (with my emphasis):
Representatives shall be apportioned among the several states according to their respective numbers, counting the whole number of persons in each state, excluding Indians not taxed. But when the right to vote at any election for the choice of electors for President and Vice President of the United States, Representatives in Congress, the executive and judicial officers of a state, or the members of the legislature thereof, is denied to any of the male inhabitants of such state, being twenty-one years of age, and citizens of the United States, or in any way abridged, except for participation in rebellion, or other crime, the basis of representation therein shall be reduced in the proportion which the number of such male citizens shall bear to the whole number of male citizens twenty-one years of age in such state.
UPDATE: Marty Lederman reminds me of language in Wesberry itself, noting “The history of the Constitution, particularly that part of it relating to the adoption of Art. I, § 2, reveals that those who framed the Constitution meant that, no matter what the mechanics of an election, whether statewide or by districts, it was population which was to be the basis of the House of Representatives.” He suggests this language as well as originalist material throws doubt on my point # 2 (that the logic of a decision in favor of the voter denominator in Evenwel logically could apply to congressional districts as well). It is a fair point, and I had forgotten about this language. Nonetheless, if indeed (as I think remains unlikely) the Supreme Court holds in Evenwel that one person, one principles in the equal protection clause require the use of total voters otherwise some voters’ votes are unconstitutionally diluted, it is hard to imagine the court not applying the same logic and denominator to congressional elections. (This is one of the reasons why, as Marty has suggested, the court is unlikely to require the use of the voters denominator on the state level in Evenwel.)