Supreme Court Justice Elena Kagan. (Mark Wilson/Getty Images)

Richard L. Hasen is the chancellor’s professor of law and political science at the University of California at Irvine and the author of “Plutocrats United.”

Sometimes the most important stuff in Supreme Court opinions is hidden in the footnotes. In Monday’s Supreme Court ruling striking down two North Carolina congressional districts as unconstitutionally influenced by race, the majority buried a doozy, a potentially powerful new tool to attack voting rights violations in the South and elsewhere.

At issue in the case was whether two congressional districts drawn by the North Carolina General Assembly were unconstitutional “racial gerrymanders.” A racial gerrymander exists when race — not other criteria, such as adherence to city and county boundaries, or efforts to protect a particular political party — is the “predominant factor” in how a legislature draws lines and the legislature presents no compelling reason for paying so much attention to race.

The more interesting of the two findings involved North Carolina’s 12th Congressional District, on which the court divided 5 to 3 in upholding a lower court’s ruling that the district was a racial gerrymander. The state had defended its line-drawing on the grounds that the meandering district was constructed that way for partisan purposes. It was all about helping Republicans, North Carolina argued — not divvying up white and black voters. The Supreme Court so far has refused to strike down districts that look like partisan gerrymanders, so proving that this was about party and not about race would create a safe legal harbor for the state.

On the surface, Justice Elena Kagan’s opinion might seem relatively modest — a win for those challenging North Carolina’s districts, to be sure, but a straightforward application of established principles about deferring to the factual findings of lower-court judges. The lower court in the case had found that race was the driving factor, and Justice Clarence Thomas — a rather surprising addition to the majority — wrote a concurrence stressing that lower courts’ factual findings are entitled to considerable deference.

But two footnotes in the case radically rework the court’s thinking about the relationship between racial and political- party discrimination in a way that should greatly expand the ability to bring gerrymandering claims in states where race and party overlap significantly.

Kagan explained that, when it comes to drawing congressional districts, race and party are not necessarily separate categories. She wrote that plaintiffs can succeed in showing that race predominated in drawing lines even if “a legislature elevated race to the predominant criterion in order to advance other goals, including political ones.” Further, she wrote that “the sorting of voters on the grounds of their race remains suspect even if race is meant to function as a proxy for other (including political) characteristics.”

This seems to be a much more realistic approach to the interrelated matters of race and party than the court’s earlier treatment of them as either-or propositions for purposes of assessing the legality of gerrymandering. Indeed, political scientists refer to places such as North Carolina as states with “conjoined polarization,” where racial and partisan categories overlap. In North Carolina, about 90 percent of black voters are Democrats; conversely, the overwhelming majority of whites are Republicans. When the Republican legislature passes a plan to limit Democratic voting power, it necessarily affects black voters.

Under this logic, legislators will no longer be able to hide behind claims of partisan motivation to protect themselves from racial gerrymandering claims. Kagan’s approach should allow voting rights plaintiffs to bring more successful racial gerrymandering claims.

And this approach has broader application — especially important given the Supreme Court’s landmark 2013 decision overturning key portions of the Voting Rights Act that had required states such as North Carolina to get federal approval before making new voting rules that could hurt minority voters: Last week, the Supreme Court declined to review a North Carolina case involving voter identification and other strict voting laws. The U.S. Court of Appeals for the 4th Circuit had also treated race and party as proxies for one another and said that a partisan-driven voting law also constituted intentional race discrimination.

Whether the race-and-party proxy approach will survive at the Supreme Court in the long term is uncertain. But at least for now, this new realism — even if buried in some footnotes — represents a welcome development.