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For two decades, a certain kind of American political thinker has insisted they know the real problem. Authoritarianism, oligarchy, and racism were symptoms rather than causes. The true pathology was partisan polarization. The sorting of Americans into hostile camps. The collapse of bipartisan comity.

We built serious institutions around this diagnosis. Duke opened its Polarization Lab. Princeton launched its Bridging Divides Initiative. No Labels raised tens of millions of dollars. Braver Angels held town halls. The Carnegie Foundation offered prestigious fellowships, and the American Academy of Arts and Sciences convened a blue-ribbon commission. Ezra Klein’s bestselling book didn’t seek to answer why democracy is dying, but Why We’re Polarized. Today there are more conferences and fellowships devoted to “bridging divides” than there are functioning bridges between the parties.

The Supreme Court just revealed where that project was leading. In Louisiana v. Callais, the court’s conservative majority held that when a legislative district is........

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