The big surprise from Monday’s Supreme Court decision in Evenwel v. Abbott was not how the Court ruled, but that it was unanimous in doing so. The case involved the way to measure population in applying the Court’s longstanding “one person one vote” standard when drawing voter districts. The basic question: should voting districts be apportioned by the number of eligible voters or by total population? The Court had never squarely answered this question.  At oral arguments last December, observers read the justices as closely divided. The challengers appeared to have a strong case. In the end, however, this was not so. Although there was some disagreement among the justices, they spoke as one in rejecting the challenger’s claim that the Constitution required apportionment to be based on eligible voters.

So what happened? Here are two hypotheses that attempt to explain why such a deeply divided Court produced no dissents in this case.

(1) Practicalities. The challengers in this case argued that when measuring voting districts under the one person, one vote standard, only eligible voters should be counted. But the relatively well entrenched norm today is to measure total residents. Some scholars have questioned whether it would even be feasible to gather accurate measures of eligible voters.  Justice Ginsburg, writing for the majority noted that to adopt voter-eligible apportionment would “upset a well-functioning approach to districting that all 50 States and countless local jurisdictions have followed for decades, even centuries.” The disruption costs and practical impediments likely pulled on the Court, even those justices who appeared sympathetic to the challengers’ claim. 

(2) Federalism. The Court did not rule that apportionment had to be done one way or another. Ginsburg’s opinion for the Court held that there was no constitutional requirement that it be based on eligible voters. The selection of what measure to use is not dictated by the Court; it is left to the states. This is a resolution that aligns with the federalism sentiments conservatives on the Court tend to embrace. By leaving the issue to the states, Justice Ginsburg probably envisions a continuation of the status quo–states, because of the practicality concerns mentioned above, will continue to rely on total population. But Justice Alito, who appears more sympathetic to the eligible-voter standard, may envision that states will respond by moving away from the total-population standard. This decision, then, left the door open for the Court to return to this issue in the future. In his concurrence, Justice Alito wrote that the Court can decide if a state is allowed to use some measure other than total population “if and when” a districting plan of that nature arises.

“By leaving open the possibility of using the population of voters as a redistricting measure in the future, Ginsburg may have won over the court’s conservative justices,” writes The National Law Journal. “The door is now wide open,” write the editors or USA Today, “for Republican legislators to change state or local redistricting policy to include only voters, and send the question back to the high court.” If indeed this issue returns to the Supreme Court at a future day, this time in the form of a constitutional challenge to a state that chooses to rely on eligible voters in measuring voter districts, then the unanimity of Evenwel is unlikely to be replicated.