Last week the Supreme Court heard oral arguments in a case called Evenwel v. Abbott. The case involves an issue of increasing importance to American politics: congressional districting. It got to the Supreme Court because conservative litigators with a successful track record of fighting against the right to vote are trying to turn the logic of pro-voter rights decisions on their head. And it's very possible that they may succeed again.

This most recent battle in the voting rights war involves two of the Warren Court's most important decisions. One of the tactics that state legislatures used to disenfranchise African-Americans was to draw district lines (or refuse to revise them) in ways that left minority voters massively underrepresented. In Alabama in 1964, for example, some counties included 40 times more people than others. In Baker v. Carr and Reynolds v. Sims, the Supreme Court held that such schemes were illegal. States were required to adhere to a "one person, one vote" standard when apportioning their legislatures. Combined with robust enforcement of the Voting Rights Act, these landmark cases helped to end Jim Crow disenfranchisement schemes.

Perversely, this lawsuit hopes to use these decisions to turn back the clock and dilute the representation of minority voters. The theory of the lawsuit is that Texas violated the Equal Protection Clause when it drew its district lines based on total population rather than on the population of voters. The state, according to the theory, should only be able to conduct apportionment according to the number of eligible voters.

If adopted, the theory presents an obvious practical problem. Total population is measured with reasonable reliability by the Census. Eligible voters are much harder to measure, not least because the numbers change every election. (What should be counted — presidential election years? Off years? State elections? Some combination?) The discretion the measure would leave to legislators leaves the process open to more of the kind of manipulation that Reynolds v. Sims tried to minimize. Plus, it just seems illogical for a state's representation in Congress to be based on total population, but its districts drawn by eligible voters.

Which brings us to the even bigger problem with the theory: In most cases, the effect of the rule change would be to overrepresent white voters and underrepresent minority voters. As Slate's Dahlia Lithwick puts it, "if the plaintiffs win this appeal, power will shift markedly from urban voters to rural voters and to white and Republican districts over minority and Democratic ones." To read the Equal Protection Clause to not merely permit but require the underrepresentation of minority voters is, to say the least, perverse.

That the argument should be indefensible doesn't mean that it can't win. The group bringing this lawsuit scored a major anti-voting rights victory with the 2013 case Shelby County v. Holder. In that case, a bare majority of the Supreme Court gutted the Voting Rights Act. Even worse, it did so by arguing that the explicit powers given to Congress to enforce the 15th Amendment were trumped by an alleged "equal state sovereignty" principle, an idea without support in the text of the Constitution or Supreme Court precedents not written by John Roberts, save for the infamous Dred Scott v. Sanford. If the Roberts Court is willing to cut the heart out of the most important civil rights statute since Reconstruction based on arguments that feeble, it's hard to imagine why they wouldn't put the interests of the Republican Party over the interests of voters in Evenwel v. Abbott.

That said, oral argument did not clearly indicate how the case will come out. The Court's Democratic nominees were predictably hostile. Anthony Kennedy, the likely swing vote, appeared curious but non-committal to the plaintiff's novel theory. Even if the Court doesn't buy the argument that the Constitution requires the states to use voters rather than total population, if it signals that this kind of districting is permitted the consequences could be dire.

This case has to be seen as part of a larger political struggle. The Republican Party faces a problem: Demographic changes are making its overwhelmingly white voter base a smaller part of the population. This year, their presidential primary, in which the major candidates try to out-xenophobe one another, will make this problem worse rather than better. To combat this, Republican states have adopted various measures to suppress minority voters — if you can't attract their votes, keep ‘em from the ballot box or try to make their votes count less through gerrymandering. Their allies in the Supreme Court might well use this case to assist in this vote-suppression effort once again.