The simple statistical error Republican Supreme Court justices used to gut the VRA
The Court says vote dilution can be proven only after "controlling" racial polarization for partisan polarization. This is a nonsensical and impossible test
Strength In Numbers is a reader-funded newsletter on politics, polling and elections from a data-driven and pro-democracy perspective. My Friday columns will always be free. Paid subscribers get my premium Tuesday Deep Dive, early access to our monthly Strength In Numbers/Verasight polls, and make it possible for me to do this work as my full-time job. Sign up for just $10/month or $90/year (a 25% discount!).
The six Republican-appointed justices on the United States Supreme Court have found a magical solution to political polarization. All you have to do is take a partisan election result and subtract out the effects of party loyalty on the result.
That, more or less, is what the Court wrote when it invalidated the Voting Rights Act last week. In Louisiana v. Callais, decided 6-3 on April 29, 2026, the conservative majority told voting-rights plaintiffs they must now “control for party affiliation” before their evidence of racial bloc voting will count under Section 2.
That sounds like a neutral statistical fix, but in reality, it’s a bad control — an error called “conditioning on a mediator variable“ that would get your paper sent back to you with lots of red ink in statistics 101. The problem is that in modern America, party isn’t a variable that operates independently of race. Rather, political party is largely downstream of one’s race. If you subtract the effects of political party from the analysis of polarization, you are subtracting away the very evidence of polarization you are trying to study!
This is important (not just a piece for nerds) because Republican legislatures are already moving ahead with new partisan and racial gerrymanders based on SCOTUS’s new theory. Tennessee passed a 9-0 GOP map this week that splits Memphis’s majority-Black and solidly Democratic 9th District into three majority-white, Republican-leaning seats. Mississippi’s governor has called a special session for May 20. Louisiana is losing at least one of its majority-Black districts. And Alabama, Georgia, and South Carolina could be next. (On this week’s podcast, David and I recap these new gerrymandering efforts that are unfolding with unprecedented haste.)
This week’s Chart of the Week is: a simple table (and one causal diagram) that shows how the Court’s new test makes racial polarization vanish on paper, while it is very much still alive in real life.
What the Court decided in Callais
To win a Section 2 vote-dilution case involving single-member districts, minority-group plaintiffs have traditionally had to clear three “preconditions” set by the Court in Thornburg v. Gingles. First, they must show that the minority community is large and compact enough to form a majority in a reasonably configured district. Second, they must show that minority voters are politically cohesive. Third, they must show that the white majority votes as a bloc often enough to defeat the minority group’s preferred candidates. Only then does the court move to the broader “totality of circumstances” inquiry.
Callais rewrites that framework in two places. On the second and third preconditions, Justice Alito says plaintiffs “must control for party affiliation” when proving minority political cohesion and white bloc voting. Failure to do so, the majority says, risks confusing partisan effects for racial ones.
Second, the Court makes it harder for plaintiffs to clear the first precondition — the illustrative-map hurdle. Plaintiffs do not merely have to show that an additional majority-minority district can be drawn in a reasonably configured way. Their proposed map must also satisfy the state’s “legitimate districting objectives,” including the state’s “specified political goals.” If the state’s goals include a target partisan distribution, a specific margin of victory for incumbents, or another constitutionally permissible goal, plaintiffs’ maps must achieve those goals “just as well.”
In other words, where the state asserts partisan advantage as one of its objectives, plaintiffs may now have to produce their own map that preserves the same partisan advantage as the map they are challenging. A state can defend a racially dilutive map as a partisan map, then require plaintiffs to draw an alternative that leaves the partisan outcome intact. As Adam Serwer wrote in The Atlantic last week, Alito and his allies have now given politicians permission to discriminate against voters, as long as they say it’s for partisan purposes. (What other major purpose would there be?)
In Callais, the Supreme Court concluded Louisiana’s second majority-Black district could not survive that test. The plaintiffs that brought the lawsuit over the state’s original racial gerrymander failed at every stage: their illustrative maps did not meet Louisiana’s nonracial goals, including political goals; their bloc-voting evidence did not control for partisan preference; and their totality-of-circumstances evidence did not show an objective likelihood of intentional discrimination.
Samuel Alito should take a stats class
In her dissent, Justice Kagan points out the nonsensical nature of this “updated” VRA. If minority citizens vote mainly for one party and majority citizens mainly for another, she writes, then under the majority’s rule “none of that difference can count” in determining whether minority voters were diluted. Plaintiffs must remove from the equation the very thing they are trying to prove: “polarized voting preferences.”
The majority’s logic assumes that race and party are two separate things that happen to be correlated — and that party can therefore be treated as a confounder. In statistics, a confounder is an outside variable that creates a relationship between two other variables. If party were truly a confounder, then controlling for it might help isolate the independent effect of race on voting behavior.
But that is not the causal story here. One’s political party is not some outside variable exerting pressure on voting in isolation of race. In modern American politics, it is often a mediator — one of the mechanisms through which race is politically consequential. Race helps shape party identification, and then party identification helps shape vote choice. So when the Court tells plaintiffs to “control for party,” it is not asking them to remove statistical noise. Instead, it is asking them to remove one of the main pathways by which racial polarization operates and is able to be measured.
That is the big error at the heart of Callais. The Court treats party as though it explains racial polarization away, when in fact party today is largely how racial polarization shows up in elections — which are, y’know, contests between parties.
This is such a basic error that the natural assumption is that the justices are engaging in bad-faith reasoning wilfully. Party identification is not merely a preference that sits next to race in a regression. And we know that party ID is largely downstream (though in some weird cases, upstream) of voters’ racial identities.
If race shapes party, and party shapes vote, then “controlling for party” is really just “ignoring the effects of race as moderated through party identity.”
The diagram below shows party as a mediating variable for vote — a variable that sits on the causal path between A and C, where A causes B and B causes C. You can see with this graph that “controlling” for political party doesn’t “isolate” the effect of race, it deletes it:
Read the diagram from left to right. In English, this chart shows that race shapes which party people identify with. Party ID then shapes their vote choice. There’s also a small direct effect of race on vote, but it’s swamped by the path that runs through party.
When the Court tells plaintiffs to “control for party affiliation,” it’s telling them to block that big middle arrow. What’s left is the small, not-party-related, direct effect of race on voting.
Now, for some real-world numbers, you can also see the pattern at work in our own polling data. I went back through a full year of Strength In Numbers/Verasight monthly polls — with nearly 13,000 respondents between May 2025 and April 2026 — and crunched the numbers on race and race-within-party vote choice in the 2024 election.
Below, you can see that Black voters in Verasight’s polling recall voting for Kamala Harris with 76% of the vote to Trump’s 24%. White voters, on the other hand, went for Donald Trump 57% to 43%. That’s a 66-point racial gap in vote choice. (Pew has the gap larger, closer to 75-80 points.)
Then, the panels on the middle and right control for party. Among Democrats, white voters split for Harris 93-7, and Black voters split for Harris 92-8. Among Republicans, white voters went for Trump 95-5, and Black voters went for Trump 88-12. Within each party, the racial gap collapses to a few points or less.
In Callais, the Court tells us to ignore differences in voting behavior on the left-hand plot and instead focus on the right-hand side. It asserts that racial polarization is a function of political polarization, not the other way around. And they arrive at this conclusion not because race stopped mattering in politics (regardless of what Thomas writes in Alexander v. South Carolina State Conference of the NAACP ), but because the Court now asks us to ignore the evidence in front of our eyes.
Kevin Morris of the Brennan Center framed this problem as a sort of Venn diagram the day the Callais decision came down. You have one blue circle for race, one yellow circle for partisanship, and a green slice of overlap in the middle. The Court’s new standard, he wrote, requires plaintiffs to “prove the green segment is not part of the yellow circle.” It can’t be done. The green segment exists because race and party are entangled.
The majority’s justices are either too ignorant of social science and statistics to realize their error (the conservative justices have decried political scientists in the past), or are blinded by the convenience of their decision for their political side. But it is still nonsense either way.
Voters, not politicians, are the real gerrymanderers
It is worth saying a few additional words about the new Callais re-reading of Gingles. Specifically, the Court now tells us to believe state legislatures and partisan map-drawers are acting in good faith, whereas plaintiffs are operating on the basis of party. Huh?
Two years ago, in Alexander, the Supreme Court flipped the burden of proof in racial gerrymandering cases. Plaintiffs no longer get the benefit of the doubt when challenging a map; legislatures do. As Justice Alito wrote for the majority then:
“A party challenging a map’s constitutionality must disentangle race and politics if it wishes to prove that the legislature was motivated by race.”
Callais now adds the new control-for-party requirement on top of that, plus the demand that plaintiffs draw an alternative map that preserves the state’s partisan goals.
Plaintiffs, in other words, are suspected of using race to draw maps that accomplish their partisan political goals from the shadows. And yet, legislatures are allowed to use party for their own partisan political goals — even when party and race are demonstrably linked!
The Supreme Court has given parties the license to pick their voters.
We are already seeing the effects of this rudimentary misunderstanding of political behavior
And don’t forget Rucho v. Common Cause (2019), which put partisan gerrymanders entirely beyond the reach of federal courts. The conditions for maximum partisan gerrymandering — super-mandering, as we called it in our podcast May 7 — are now satisfied: Legislatures get the presumption of good faith, plaintiffs get a statistical test they can’t pass, and the partisan motive a state is allowed to claim is itself practically unreviewable (as Kagan points out, it is foolish to think any partisan map-drawer would leave “smoking gun” evidence of intent to racially discriminate around for the courts to discover).
And this week, we got the first look at what this statistical error has cost voters.
Tennessee Republicans on Thursday, May 7, 2026 passed a new congressional map that gives the GOP all nine of the state’s House seats. The current 9th District, anchored in Memphis, is 60% Black by voting-age population and voted for Harris 71-27. Under the new map, the 9th is 38% Black VAP and would have voted Trump 60-39. As shown in this chart from The Downballot:
Worse, as a politically salient feature of Tennessee, Memphis has basically been removed from the map. The city’s voters are now “cracked” between three seats, each grouped in with enough white Republican suburbs and exurbs to drown the Black vote in a partisan tide.
Pre-Callais, Tennessee’s map would have faced a Section 2 suit on day one, and the plaintiffs would have had a real chance. Post-Callais, the state’s defense will simply be that lawmakers weren’t sorting by race, but party — the Black voters just so happen to be Democrats.
The same process is now unfolding in at least 6 Southern states simultaneously.
The death of districts and super-mandering
In no simpler terms, the Roberts Court’s new test for VRA Sec. 2 vote dilution cases makes racial polarization harder to prove, and invites politicians to sort congressional district plans by party. A state with a history of discriminating against minority voters can now do so — is in fact welcomed to, via the confluence of Callais and Rucho — as long as it claims it’s targeting Democrats instead, thank you very much.
This is the result of a simple statistical error. And yet, the consequences will be anything but simple. Under the guise of partisan rigging, it will now be simple for state legislators to rig their federal representatives for the dominant party. As I explained on this week’s podcast, even a simple computer redistricting program can efficiently sort Americans into buckets that maximize partisan edge — we’re talking a D+10 or R+10 state controlling 90% of the districts in a state:
The charts above come from a computerized redistricting simulator I built to show how trivial it would be for the dominant party in a 55-45 state to gerrymander the hell out of their map if various restrictions on things like compactness and contiguousness (constraints that vary by state and in how firmly they are implemented) are relaxed, and partisan sorting is overlooked.
With Callais, it is now easier than ever to see that the deeper problem in electoral representation today isn’t representatives or Supreme Court justices acting in bad faith, but the system itself. The problem is single-member districts. When representation depends on which voters are bundled together on a map, the party holding the pen can turn geography into power with astounding efficiency.
But that’s a piece for another time. For now, the takeaway is rather short and unhappy. The Supreme Court has decided that if you can’t see racially polarized voting in a regression — particularly one that has been designed to mask racially polarized voting — it doesn’t legally exist. Voters in Memphis will beg to differ.
Thus confirms what I’ve long known about the court. We should all be clear: these are not,stupid people, they did,this on purpose
The Court has been clear that it believes the states get to do whatever they want to determine their Congresspeople. I disagree but so be it. Gerrymandering sucks regardless of whether it is about partisanship or race. My neighborhood is an appendage to a Congressional district that resulted from a gerrymander to create a majority-minority district (seat held by Pressley which surrounds me but doesn't include me). In fact if Greater Boston were un-gerrymandered there would likely be more minority representation in Congress with no impact on D vs R.